The first shots sailed past Iraqi police officers at a checkpoint. They took off in three squad cars, their lights flashing.

It was early in the Iraq war, Dec. 22, 2004, and it turned out that the shots came not from insurgents or criminals. They were fired by an American private security company named Custer Battles, according to an incident report in an archive of more than 300,000 classified military documents made public by WikiLeaks.

The company’s convoy sped south in Umm Qasr, a grubby port city near the Persian Gulf. It shot out the tire of a civilian car that came close. It fired five shots into a crowded minibus. The shooting stopped only after the Iraqi police, port security and a British military unit finally caught up with the convoy.

Somehow no one had been hurt, and the contractors found a quick way to prevent messy disciplinary action. They handed out cash to Iraqi civilians, and left.

The documents sketch, in vivid detail, a critical change in the way America wages war: the early days of the Iraq war, with all its Wild West chaos, ushered in the era of the private contractor, wearing no uniform but fighting and dying in battle, gathering and disseminating intelligence and killing presumed insurgents.

There have been many abuses, including civilian deaths, to the point that >the Afghan government is working to ban many outside contractors entirely.

The use of security contractors is expected to grow as American forces shrink. A July report by the Commission on Wartime Contracting, a panel established by Congress, estimated that the State Department alone would need more than double the number of contractors it had protecting the American Embassy and consulates in Iraq.

Contractors were necessary at the start of the Iraq war because there simply were not enough soldiers to do the job. In 2004, their presence became the symbol for Iraq’s descent into chaos, when four contractors were killed in Falluja, their bodies left mangled and charred.

Even now — with many contractors discredited for unjustified shootings and a lack of accountability amply described in the documents — the military cannot do without them. There are more contractors over all than actual members of the military serving in the worsening war in Afghanistan.

The archive, which describes many episodes never made public in such detail, shows the multitude of shortcomings with this new system: how a failure to coordinate among contractors, coalition forces and Iraqi troops, as well as a failure to enforce rules of engagement that bind the military, endangered civilians as well as the contractors themselves. The military was often outright hostile to contractors, for being amateurish, overpaid and, often, trigger-happy.

Contractors often shot with little discrimination — and few if any consequences — at unarmed Iraqi civilians, Iraqi security forces, American troops and even other contractors, stirring public outrage and undermining much of what the coalition forces were sent to accomplish.

The mayhem cropped up around Iraq, notably in one episode reported in March 2005 in which a small battle erupted involving three separate security companies.

At a notoriously dangerous checkpoint on the main road to the Baghdad airport, a cement truck entered a lane reserved for Department of Defense vehicles. A guard from Global, a British company, fired a warning shot, and when a man initially identified as an Iraqi opened the door and tried to flee, guards from a tower started firing, too. The man dropped to the ground. Then members of an Iraqi private security team parked nearby also opened fire, shooting through the chest not the driver but a worker from DynCorp International, an American security company.

When the truck driver was finally questioned, he turned out to be a Filipino named José who worked with yet a third company, KBR, the American logistics and security giant.

The conclusion drawn from this chaos was, “IT IS BELIEVED THE DRIVER ENTERED THE DOD LANE BY ACCIDENT.”

For all the contractors’ bravado — Iraq was packed with beefy men with beards and flak jackets — and for all the debates about their necessity, it is clear from the documents that the contractors appeared notably ineffective at keeping themselves and the people they were paid to protect from being killed.

In fact, the documents seem to confirm a common observation on the ground during those years in Iraq: far from providing insurance against sudden death, the easily identifiable, surprisingly vulnerable pickup trucks and S.U.V.’s driven by the security companies were magnets for insurgents, militias, disgruntled Iraqis and anyone else in search of a target.

Most of the documents are incident reports and match what is known of the few cases that have been made public, although even this cache is unlikely to be a complete record of incidents involving contractors. During the six years covered by the reports, at least 175 private security contractors were killed. The peak appeared to come in 2006, when 53 died. Insurgents and other malefactors kidnapped at least 70 security contractors, many of whom were later killed.

Aegis, a British security company, had the most workers reported killed, more than 30. Most of those were Iraqi drivers, guards and other employees. Not only the military, but journalists and aid workers as well relied on contractors to help protect them.

The security contractors seemed overmatched, often incinerated or torn apart by explosions their vehicles had no chance of warding off. In August 2004, the corpses of two men who had worked with Custer Battles were found charred and abandoned in a truck that was still burning on the road between Tikrit and Mosul, after it was struck by an improvised explosive device and fired upon from a Volkswagen, one report said.

In July 2007, another report said, two were killed when a gun truck operated by ArmorGroup, a British company, flew like a wobbling discus 54 yards through the air, flipping approximately six times, after a huge I.E.D. exploded beneath it in northern Iraq.

And in May 2009, three Americans, including a senior Navy officer, were killed outside Falluja when an I.E.D. overturned a vehicle escorted by Aegis contractors during a visit to a water treatment plant financed by the United States, according to another report and American government statements at the time.

Death came suddenly, from all sides, in all forms.

In late 2004 in Tikrit, seven men emerged from two Daewoo vehicles and mowed down Iraqi workers for Buckmaster, a company hired to destroy old munitions, as the workers got out of a bus, a report said. The gunmen did not flee until they ran out of ammunition, killing 17 and wounding 20 as two Iraqis saved themselves by hiding under seats in the bus.

There were suicide bombings, desert ambushes, aviation disasters and self-inflicted wounds, as when a Ugandan guard working for EOD Technology, an American company, shot and killed his South African supervisor and then himself in 2008 after being terminated, a report said.

A spokesman for EOD confirmed the incident and said that the investigation had been unable to determine “why this particular guard decided to take the actions that he did.”

“I think the only elaboration on this incident is to note that it was a very sad and unfortunate event,” said the spokesman, Erik S. Quist.

In another case, in Baghdad in the summer of 2009, a British contractor with ArmorGroup was reported to have shot and killed two co-workers, a Briton and an Australian, then run wild through the heavily fortified Green Zone in an attempt to escape. Finally, a coalition soldier tackled him, a report said, and another soldier “shot a directed-aimed warning shot into sand bags which immediately stopped resistance from suspect so that he could be brought under control.”

The alleged killer, Daniel Fitzsimons, is still being held in Baghdad while awaiting trial under Iraqi law.

The contractors also suffered horrific traffic accidents with multiple fatalities all over Iraq, seemingly as a side effect of driving at high speeds on bad roads where a threat can appear at any moment.

The threats were not limited to insurgents, the documents show: private security contractors repeatedly came under fire from Iraqi and coalition security forces, who often seemed unnerved by unmarked vehicles approaching at high speeds and fired warning shots, or worse. Even as the war dragged on, there seemed no universal method for the military to identify these quasi soldiers on the battlefield.

To cope, the contractors were reduced to waving reproductions of coalition flags from inside their vehicles, the documents show — but even that did not always work. After being shot at by an American military guard tower near Baiji in July 2005, contractors with Aegis first waved a British flag. When the shooting continued, the contractors, who said they were transporting a member of the American military at the time, held up an American flag instead. “THE TOWER KEPT SHOOTING,” a report said, although no one was injured in the episode.

But whatever the constellation of reasons — from war-zone jumpiness to outright disregard for civilian lives — the security companies are cited time after time for shootings that the documents plainly label as unjustified. This has blackened their reputation, even if it has not lessened the military’s dependence on them. “AFTER THE IED STRIKE A WITNESS REPORTS THE BLACKWATER EMPLOYEES FIRED INDISCRIMINATELY AT THE SCENE,” read one report from Aug. 22, 2006, referring to the company, now known as Xe Services, that the following year would become notorious for an apparently unprovoked killing of 17 Iraqis at Nisour Square in Baghdad.

In a written statement last week, Xe said, “While it would be inappropriate to comment on specific cases, we work closely with our government customers and cooperate fully in all investigations.”

In December 2004, just a few days after the confrontation with Iraqi security forces, another Custer Battles convoy fired into the windshield of a Humvee driven by American military police soldiers in a patrol that was approaching the convoy from behind on another road near Baghdad. The report noted laconically that the security contractors did not stop their convoy until they reached an American checkpoint, “WHERE THEY ADMITTED TO FIRING ON THE MP PTL,” the military police patrol.

Many of the companies apparently felt no sense of accountability. Contractors with a Romanian company called Danubia Global killed three Iraqis in Falluja in 2006, another report said, then refused to answer questions on the episode, citing a company policy not to provide information to investigators.

In 2007, a convoy operated by Unity Resources Group, based in Dubai, shot at an approaching vehicle near the Green Zone in Baghdad, wounded a bodyguard for President Jalal Talabani of Iraq and did not report the shooting until Mr. Talabani’s staff contacted the American authorities, one report said.

When asked about the incident last week, a Unity official, Jim LeBlanc, said that “in a time of numerous suicide vehicle attacks, a vehicle had presented itself in a profile that was consistent with the behavior of a suicide attacker.” Unity guards fired “carefully aimed warning shots” when the vehicle refused to stop, Mr. LeBlanc said, and the company did not initially believe that anyone had been hurt.

Only when contacted by American investigators did Unity realize that “an Iraqi security force member” had been struck by a ricochet, and from that point on, the company fully cooperated, Mr. LeBlanc said. After the investigation, he said, “all Unity members were cleared to immediately return to work.”

And still more recently, in July 2009, local contractors with the 77th Security Company drove into a neighborhood in the northern city of Erbil and began shooting at random, setting off a firefight with an off-duty police officer and wounding three women, another report said.

“It is assessed that this drunken group of individuals were out having a good time and firing their weapons,” the incident report concluded.

In many other cases, contractors cited what they considered a justifiable “escalation of force” as an Iraqi vehicle moved toward them and did not respond to “hand signals” and other signs that the driver should stop. At that point, the contractors would fire into the vehicle’s engine block or through the windshield.

The Iraqis who were shot at, and who the documents show were nearly always civilians, not surprisingly saw things differently. To judge by the disgust that seeps through even the dry, police-blotter language of some of the incident reports, American military units often had a similar perspective. That appears to be especially true of reports on “escalations of force” by Blackwater in the years leading up to the Nisour Square shooting, the documents show.

On May 14, 2005, an American unit “OBSERVED A BLACKWATER PSD SHOOT UP A CIV VEHICLE,” killing a father and wounding his wife and daughter, a report said, referring to a Blackwater protective security detail.

On May 2, 2006, witnesses said that an Iraqi ambulance driver approaching an area struck by a roadside bomb was killed by “uncontrolled small arms firing” by Blackwater guards, another report noted.

On Aug. 16, 2006, after being struck by an I.E.D. in the southbound lane of a highway, Blackwater contractors shot and killed an Iraqi in the back seat of a vehicle traveling in the northbound lane, a report said. At least twice — in Kirkuk and Hilla — civilian killings by Blackwater set off civilian demonstrations, the documents say.

And so it went, up to the Sept. 16, 2007, Nisour Square shooting by Blackwater guards that is again noted as an “escalation of force” in the documents. Little new light is shed on the episode by the documents, although in a twist, the report indicated that the street from which the Blackwater convoy charged into the square went by the military code name Skid Row.

The last reference to Custer Battles, which eventually lost a $10 million whistle-blower case in which it was claimed that the company defrauded the United States on billing invoices for the company’s work in Iraq, appears in a report dated March 15, 2005, describing an I.E.D. strike on an exit ramp in western Baghdad. An Iraqi driver for the company received shrapnel wounds in the face from the bomb and was wounded in the chest by gunfire that broke out after the explosion. The driver was taken to a local hospital, ultimate fate unknown.