While officials found no single factor that prompted Spc. Ivan Lopez to go on a shooting rampage April 2, the Army's investigation found instances where he faced "significant and increasing stress in the days, weeks, and months leading up to the shooting."
Army officials also would have had difficulty recognizing any personal problems leading up to the attack because risk assessment relies on self-reporting and Lopez had been dishonest with his supervisors, the report said.
But it also highlighted gaps in information sharing, noting that the 34-year-old's supervisors in some cases thought federal medical privacy laws prevented them from obtaining information to which they should have had access.
After the 2009 Fort Hood massacre, the Army made 78 recommendations for the base to improve its ability to identify the potential risk of violent behavior. In Friday's report, the Army says there was one recommendation it did not implement - better sharing of soldiers' health history - because of constraints on exchanging information between military and civilian behavioral health care providers.
"In the absence of a system capable of identifying (Lopez) as a threat, and because the unit was unaware and unable to address the variety of stressors in (Lopez's) life, Fort Hood was not able to prevent the shooting," lead investigator Lt. Gen. Joseph E. Martz said.
Investigators have said the Iraq War veteran was undergoing treatment for depression and anxiety while being evaluated for post-traumatic stress disorder, but was not considered "likely" to commit violence. The report also said two family deaths and financial problems could have had a cumulative effect on Lopez's emotional state.
A spokesman for Lopez's family said in April that he was upset he was granted only a 24-hour leave - which was extended to two days - to attend his mother's funeral in Puerto Rico. Army spokesman Lt. Col. Donald Peters said it is "absolutely untrue" that he was granted only a day's leave, instead getting six days' leave.
Neither Lopez's wife nor the family spokesman could be immediately reached.
Almost half of Lopez's monthly salary as an Army specialist was going to loan repayments and child support, the report said.
Lopez had been seeing a doctor monthly at Fort Bliss, was transferred to another base for four months and then arrived in Fort Hood last February. Officials at Fort Bliss did not appear to schedule a doctor's appointment for Lopez at Fort Hood, as Army policy would have required.
"We find insufficient evidence to conclude that leaders and medical providers at Fort Bliss or Fort Hood could have affected (his) decision to shoot his fellow soldiers," the report said. "Any other conclusion is simply conjecture."
Risk assessments rely upon self-reporting, according to the Army report, which said Lopez could sometimes be "misleading or deceptive." To that end, military officials also found a Facebook page on which Lopez falsely claimed to be an Army sniper who had been to the Central African Republic.
The Army previously said it was logistically impossible to stop and search all 80,000 people who work on the sprawling base every day.
Recommendations in the report, for which more than 160 witnesses were interviewed, included exploring whether soldiers should register privately owned weapons with their commanders. Lopez flashed his badge to enter the base and carried out the shooting with a .45-caliber Smith and Wesson that was not registered with the installation.
Lopez's attack occurred nearly five years after Army psychiatrist Nidal Hasan gunned down 13 people at a medical readiness building at Fort Hood. Hasan carried a high-powered pistol and several cartridges of ammunition into the building under his green Army fatigues and opened fire for several minutes.
An FBI review found that authorities missed several messages Hasan had sent to a Yemen-based cleric tied to terrorist activity. Evidence presented at his trial two years ago included testimony that he had trained to quickly fire the pistol at a nearby gun range.
Fort Hood officials remain committed to "doing what's necessary to ensure the safety and security of all personnel," the Army base said in a statement Friday.
Associated Press writers Lolita Baldor in Washington, D.C., and Nomaan Merchant in Little Rock, Arkansas, contributed to this report.