Seventeen US soldiers from a Colorado military base who mostly served in Iraq have been linked to violent killings and attempted killings since their return to US soil. Three of them came from one platoon - highlighting how a generation of American soldiers are struggling to cope with life after military service.

"I was having a total mental breakdown. Every day we were getting in battles, and never having a break, it seemed like, it was just crazy.

Image caption Four members of the Third Platoon are now in prison after serving in Iraq

"I just got to where I couldn't take it. I tried to go to mental health, and they put me on all kinds of meds, too. And I was still going out on missions... they tried different medications, different doses, and nothing worked."

Kenny Eastridge was a decorated gunner, but is now serving 10 years in prison for his role in the murder of fellow soldier Kevin Shields in Colorado Springs.

In November 2007, Eastridge along with two other soldiers, Louis Bressler and Bruce Bastien, were out drinking in a nightclub with Mr Shields after returning from a rough combat tour in Baghdad.

Drunk and stoned, they drove off to find more alcohol. Minutes later, Specialist Kevin Shields lay dead, gunned down in a drunken argument, and left in a pool of blood by the side of the road.

Bressler and Bastien were sentenced to 60 years in prison for the murder and a string of other crimes in Colorado Springs.

Kevin Shields' murder was not a unique case. At Fort Carson military base, 17 soldiers have been charged or convicted of murder, attempted murder or manslaughter in the past four years.

In the first six months you're just happy to be home. And then after that... problems started Ryan Krebbs, Third Platoon medic

For over a year, This World has been tracking down the members of Third Platoon, Charlie Company, 1st battalion, 506th infantry, which later reflagged and became the 2nd batallion, 12th infantry regiment, trying to make some sense of the killings that have occurred since their return to the US.

The majority of Third Platoon served multiple combat tours with distinction and managed to adjust to life after Iraq. But a significant minority have not.

Four of the platoon have ended up in prison. Two are dead - one died from an overdose, another was killed by a suicide bomb.

In all, 15 out of 42 soldiers from Third Platoon left the army after a single Iraq tour. Four were kicked out for failing drug tests, and one was sent to prison for driving while drunk and fleeing the scene of an accident. Five were medically discharged. Only five left the army because their service had ended.

More than half of the platoon said they suffered from psychological problems after Iraq.

'Trigger happy'

The platoon's youngest member, Jose Barco, is serving 52 years in jail for shooting and wounding a pregnant woman when he opened fire at a party in Colorado Springs. He was convicted on two counts of attempted murder.

Barco said he became desensitised to death and killing during the vicious combat of the "surge" in 2007, when his battalion were tasked with driving al-Qaeda out of Baghdad.

It was Third Platoon's job to move mutilated bodies every morning.

"It got to the point where it was like seeing a dead dog or a dead cat. If you're not numb in those moments, you're going to go crazy. I guess it just follows me," he said from his prison cell.

As Third Platoon's tour wore on, discipline deteriorated. Jose Barco said that for some soldiers, casual brutality became the norm, and that he routinely shot unarmed Iraqis.

"We were trigger happy. We'd open up on anything. They even didn't have to be armed. We were keeping scores," he said.

The US army investigated, but no soldier from Third Platoon has been charged with killing civilians in Iraq.

While in Iraq, Eastridge had exhibited signs of post-traumatic stress disorder (PTSD), was taking anti-depressants and sleeping pills, but was also taking valium, smoking pot and drinking whisky.

He had a history of aggression, and been charged with assault before he went on his second tour, but he was still deployed.

He said Iraqi civilian deaths did not bother him at all: "You disassociate. To you they're not even people, you know. Like, they're not humans."

Third Platoon's Iraq tours: • August 2004 - August 2005. Stationed in the desert in the heart of the Sunni triangle, they patrolled the main highway from Ramadi to Falluja drawing out insurgent attacks and dealing with 1000 Improvised Explosive Devices (IEDs) • September 2006 - December 2007. Stationed near Al Dora, an al-Qaeda "hub" that became the site of al-Qaeda's last stand in Baghdad. Main task was to try to secure neighbourhoods street by street, which were subject to widespread sectarian killings

The platoon's first battalion commander Colonel David Clark, accepted that the price of "success" on the battlefield could take a psychological toll.

"It's got to have an impact," he said.

"Is that a reason not to do the surge? No. The surge worked. We needed to do the surge. War is a dangerous thing," he added.

The number of Fort Carson soldiers failing drug tests rose by 3000% in the first three years of the Iraq war.

Ryan Krebbs, the platoon medic, admitted abusing medication in Iraq, stockpiling sleeping pills to calm himself down after missions.

He never forgave himself for the death of one of his sergeants, and eventually tried to kill himself with an overdose of prescription anti-psychotic drugs when he returned home.

"In the first six months you're just happy to be home. And then after that... problems started.

The black box warning for these anti-depressants say that they can make people suicidal Dr Joseph Glenmullen, Psychiatrist, Harvard Medical School

"Depression, anxiety, paranoia, getting the feeling that you're in Iraq all over again.

"I just couldn't take it anymore," he said.

Before the Iraq war, American soldiers on psychiatric medications were not allowed to deploy to a combat zone.

But by the time of the surge, more than 20,000 US troops in Afghanistan and Iraq were taking anti-depressants and sleeping pills to cope with the stresses of combat.

The military has come under fire for medicating troubled soldiers rather than taking them away from the front line.

Dr Joseph Glenmullen warned that such medication could be dangerous in war.

"All of these anti-depressants now carry in recent years a black box warning.

"The black box warning for these anti-depressants say that they can make people suicidal and a variety of other side effects that include insomnia, anxiety, agitation, irritability, hostility, impulsivity and aggression, all of which obviously could become critical in a combat situation," he said.

The vice-chief of the US army, General Peter Chiarelli defended the policy, but said that the army needed every soldier it could get.

"It's a supply and demand problem," he said.

Image caption More than 20,000 US troops in Afghanistan and Iraq were taking anti-depressants and sleeping pills

"I cannot do anything about the demand, I only have a finite supply, and when the demand goes up, and orders are given, we provide the soldiers."

Spurred by the public outrage, the army's medical command last year conducted an investigation in to the violence.

It found that most of the soldiers had experienced unusually intense combat in Iraq, six of them had criminal records before they joined the military, 11 had a history of substance abuse and nine were taking psychiatric medications.

It concluded that the intensity of battle and shortcomings in mental health treatment may have converged with "negative outcomes" such as alcohol and drug abuse.

Last week the final American combat brigade pulled out of Iraq after more than seven years of war.

But for many soldiers, the end of combat operations is just the beginning of a different kind of struggle back home.

This World: The Wounded Platoon will be broadcast on Wednesday, 25 August 2010 at 2100 BST on BBC Two. Or catch up afterwards on BBC iPlayer.