Six other slayings involving unit soldiers occurred in Colorado and other states since 2005.
"This investigation suggest a possible association between increasing levels of combat exposure and risk for negative behavioral outcomes," the study said.
Army investigators compared the Fort Carson unit of about 3,700 soldiers with a similarly sized unit and found it suffered more combat deaths in Iraq and was deployed there longer.
"This deployment experienced higher levels of combat intensity," the report said, adding that the soldiers also faced "significant disruptions in family-social support."
Lt. Gen. Eric B. Schoomaker, the Army's surgeon general, said Wednesday the unit's crime cluster appeared to be unique among Army bases and that its combat exposure and length of deployments are just two factor officials are looking at.
"We're starting to look into the deployments and ... how it's related with attitudes and behavior," Schoomaker said.
The accused soldiers also were at risk of violence because of prior criminal activity, drug and alcohol abuse, and mental health issues, according to Schoomaker, Lt. Gen. Michael D. Rochelle, the Army's deputy chief of staff, and Army West Division Commander Maj. Gen. Mark Graham.
Task force members suggested the Army find a way to identify soldiers who have been exposed to fierce combat. But in a bid to put the cluster in perspective, Rochelle stressed that, between 2004 and 2008, 2,726 Army soldiers were involved in violent crimes, out of a population of 1.1 million.
Nationally, at least 121 Iraq and Afghanistan war veterans have committed a killing in the United States or been charged in one.
They also recommended better training for commissioned and noncommissioned officers to manage soldiers with behavioral problems and ensure soldiers who seek help aren't humiliated or belittled.
Investigators focused on the cases of 14 soldiers accused of murder, manslaughter, attempted murder and aggravated assault, mostly with firearms.
Two of those 14 soldiers were not deployed. Among the 12 who were, investigators found the accused had experienced heavy combat in Iraq and that half of those interviewed reported witnessing war crimes, including the killing of civilians.
Schoomaker stressed Wednesday that an Army probe did not substantiate the soldiers' reports of war crimes.
Back home, the soldiers carried weapons with them because they felt "naked" and unsafe and had difficulty transitioning to civilian life. Some said they felt "weird" and didn't fit in, the Army report said.
"There, we were the law; here, the cops are the law," one of the accused told investigators. The Army report says the accused claimed their commanders and fellow soldiers did not encourage them to seek help at home.
The 4th BCT experienced a combat death rate of 8.9 per 1,000 soldiers during a first Iraq deployment and 9.6 per 1,000 on a second deployment. In comparison, the other, unidentified unit had death rates of 0.4 and 2.1 per 1,000, respectively.
The Colorado slayings include the June 6, 2008, deaths of a man and a woman gunned down by a man with AK-47 assault rifle as they put up garage sale signs on a street.
Pfc. Jomar Dionisio Falu-Vives faces first-degree murder charges in the shootings. He lived nearby and told friends he liked hearing the sirens as authorities raced to the scene, according to the Army report.
In May, Thomas Woolly, a Fort Carson soldier and Purple Heart recipient, was arrested on suspicion of manslaughter in the slaying of a 19-year-old woman. Woolly was in Fort Carson's Warrior Transition Unit, which provides support for soldiers returning from combat who were injured or have psychiatric disorders.
The spate of killings prompted then-U.S. Sen. Ken Salazar, now interior secretary, to ask the Army last year to investigate the killings.
Wednesday's study comes as the Army struggles with other combat-related issues, including increased rates of post-traumatic stress syndrome and soldier suicides.
A study last year by the RAND Corp. research organization estimated nearly 20 percent of returning veterans, or 300,000 people, have symptoms of PTSD or major depression.
Army suicides have increased yearly since 2004 as soldiers deal with longer and repeated tours in Iraq and Afghanistan. Eight soldiers at Fort Campbell, Ky., have killed themselves this year, and the Army has made suicide prevention training mandatory for soldiers and leaders.
