Last Updated Apr 3, 2014 7:54 PM EDT
FORT HOOD, Texas -- The man who gunned down three fellow soldiers at Fort Hood and wounded several others before killing himself suffered from mental illness and may have argued with other soldiers before the rampage, the base's commander said Thursday.
The shooter in Wednesday's attack -- which occurred at the same Texas military facility where an Army psychiatrist embarked on a massacre in 2009 -- was identified as Spc. Ivan. Lopez, 34, originally from Puerto Rico.
"There may have been a verbal altercation with another solider or soldiers and there is a strong possibility that may have immediately preceded the shooting," Milley said.
The general did not say what sort of dispute may have occurred, and he said there was no evidence that Lopez deliberately targeted his victims.
Lopez's unstable mental health is believed to be the underlying cause of the attack, Milley said. The general said Wednesday night that shooter had been undergoing treatment for depression and anxiety and was taking medication, including the sleep aid Ambien.
Although no diagnosis had been made, Lopez was also being evaluated to determine whether he had post-traumatic stress disorder, Milley said.
Investigators have also been told that Lopez may have been upset because the Army had refused to grant him personal leave, CBS News has learned. A family spokesman in Puerto Rico told The Associated Press that Lopez had been upset when he was granted only two days' leave to attend the funeral of his mother, who died of a heart attack in November.
An Iraq war veteran, Lopez claimed that he had suffered a traumatic head injury. If he did sustain such an injury, it was not in combat.
The soldier saw a psychiatrist last month and showed no "sign of any likely violence either to himself or others," McHugh said. His record shows "no involvement with extremist organizations of any kind."
"We're not making any assumptions by that. We're going to keep an open mind and an open investigation. We will go where the facts lead us. And possible extremist involvement is still being looked at very, very carefully. He had a clean record in terms of his behavior," McHugh said.
The shootings took place at a medical facility and a nearby command building - areas on the base that were familiar to Lopez.
Investigators searched the soldier's home Thursday and questioned his wife, said Fort Hood spokesman Chris Haug. The couple had a 2-year-old daughter and Lopez also had two children from a previous marriage, a friend said.
Lopez apparently walked into a building Wednesday afternoon and began firing a .45-caliber semi-automatic pistol. He then got into a vehicle and continued firing before entering another building.Milley told reporters military personnel tried to protect their colleagues.
"There's at least one chaplain who shielded and saved other soldiers, broke some windows and got them to safety," the general said.
One of those slain, Sgt. Timothy Owens of Effingham, Ill., was a counselor who tried to talk down the shooter, according to his mother. Owens was shot five times at close range.
The rampage ended when Lopez was confronted in a parking lot by a military police woman.
"He put his hands up and then put his hands under his jacket. He pulled out the .45-caliber Smith & Wesson weapon, at which time she exchanged fire with him," Milley said.
Lopez then put his gun to his his and pulled the trigger a final time, the general said.
A U.S. official with knowledge of the investigation told CBS News that Lopez bought his weapon, a Smith and Wesson MP 45, at Guns Galore, the same shop where Maj. Nidal Hasan, who carried out a deadly rampage at Fort Hood in 2009, bought his weapon.
Lopez purchased his weapon legally March 1.
first Fort Hood shooting in 2009 helped prevent Wednesday's shooting from being a bigger disaster. The alert system and training for first responders "contributed to making this something that could have been much worse," Odierno said during his testimony before the Senate Armed Services Committee on the Army's budget.
CBS News national security correspondent David Martin reports that Lopez joined the National Guard in Puerto Rico in 1999 and that after 15 years in the armed services had only risen to the low rank of specialist.
Sources say a preliminary check of Lopez's background has revealed no significant criminal record, and a search of the criminal and terror databases has turned up no evidence, so far, that he was connected to any kind of terrorist group, but nothing has been ruled out.
McHugh said the soldier served on a yearlong peacekeeping mission in Egypt's Sinai Peninsula in the mid-2000s. He then enlisted with the Army in 2008, McHugh said.
His weapon recently was purchased locally and was not registered to be on the base, Milley said.
Suzie Miller, a 71-year-old retired property manager who lived in the same apartment complex as Lopez near Fort Hood, said few in the area knew him and his wife well because they had just moved in a few weeks ago with their toddler.
"I'd see him in his uniform heading out to the car every morning," Miller said. "He was friendly to me and a lot of us around here."
Those injured Wednesday were taken to the base hospital and other local hospitals. At least three patients were listed in critical condition Thursday, but were expected to survive.
The shootings immediately revived memories of the 2009 shooting rampage on Fort Hood, the deadliest attack on a domestic military installation in U.S. history. Thirteen people were killed and more than 30 were wounded.
Hasan, an Army psychiatrist, was convicted last year for the November 2009 mass shooting at Fort Hood. According to trial testimony, he walked into a crowded building, shouted "Allahu Akbar!" - Arabic for "God is great!" - and opened fire. The rampage ended when Hasan was shot in the back by base police officers.
Hasan, now paralyzed from the waist down, is on death row at the military prison at Fort Leavenworth in Kansas. He has said he acted to protect Islamic insurgents abroad from American aggression.
After that shooting, the military tightened base security nationwide. That included issuing security personnel long-barreled weapons, adding an insider-attack scenario to their training, and strengthening ties to local law enforcement. The military also joined an FBI intelligence-sharing program aimed at identifying terror threats.
In September, a former Navy man opened fire at the Washington Navy Yard, leaving 13 people dead, including the gunman. After that shooting, Defense Secretary Chuck Hagel ordered the Pentagon to review security at all U.S. defense installations worldwide and examine the granting of security clearances that allow access to them.