(CNN) -- Suspected Fort Hood shooter Spc. Ivan Lopez had recently asked to transfer, claiming he was being taunted and picked on by soldiers in his unit, a senior U.S. official familiar with the investigation told CNN.
Lopez is accused of taking his own .45-caliber handgun onto the sprawling Army facility, killing three people and injuring 16 more before taking his own life last week.
Three patients are still being treated at Scott & White Memorial Hospital. All three are listed in fair condition and are continuing to improve, the hospital said Monday.
In an effort to determine what led Lopez to open fire, the Army is not only looking into his mental health history but also investigating his interactions with fellow soldiers in his unit at Fort Hood, according to a second source -- a U.S. military official directly familiar with the investigation.
Lopez joined the 154 Transportation Company at Fort Hood after arriving at the post in February. But he apparently felt some in the unit had not treated him appropriately, that official said.
The military official added that it's not clear whether members of Lopez's unit, including his supervisor, knew of his mental health history or that he was being treated for anxiety and depression.
Officials have confirmed Lopez received some initial mental health treatment at his previous post at Fort Bliss and was prescribed medications there.
Medical examiners will now try to determine exactly what medications he took, whether he took them correctly and whether there may have been some interaction between drugs.
Records show Lopez had had regular mental health appointments in recent months. But what is not known, the military official said, is what role, if any, his mental health situation or his medications may have played in his mounting anger and final attack.
Chris Gray, spokesman for U.S. Army Criminal Investigation Command, spoke to reporters Monday, outlining what investigators know so far about what happened the day of the shooting.
"We can confirm that the alleged shooter, Spc. Ivan Lopez, was involved in a verbal altercation concerning his request for leave, and the processing of that request at his unit's administrative office," the spokesman said.
Within minutes of that altercation, Lopez allegedly fired multiple rounds, killing one soldier and injuring 10. He then got into his vehicle, from which he fired his weapon at two soldiers, wounding one, Gray said.
Lopez is then believed to have entered the building where he was assigned and worked, which encompasses the unit's motor pool office and vehicle bay area. There, he opened fire again, killing one solider and injuring two more, the spokesman said.
Lopez then returned to his vehicle. While driving, he allegedly fired into another moving vehicle, striking and wounding the passenger. He also fired and wounded another solider, Gray said.
Lopez is then believed to have entered the main entrance of the medical building, killing a solider on duty and wounding another.
"At this point, we do not know why he entered that building and we may never know why," the spokesman said.
Lopez returned to his vehicle and drove into a parking lot, where he approached a responding Fort Hood military police officer.
"There was a verbal exchange between the officer and the subject. The military police officer drew her weapon and fired one round when the subject reportedly brandished his weapon," Gray said.
Autopsy results confirm Lopez was not struck by the officer's fire. The suspected shooter then allegedly turned the gun on himself.
The spokesman said there is still no known motive for the shooting. Investigators have collected no evidence to suggest it was connected to any terrorist organization, though they have not completely ruled out the possibility, the spokesman said.
Gray continued: "We sincerely hope -- all of us in law enforcement -- that our efforts to diligently seek the truth will in some small way provide comfort to the loved ones of the deceased and wounded, who are struggling through this difficult time."
President Barack Obama and first lady Michelle Obama are scheduled to attend a memorial service for the victims Wednesday.
The ceremony will be held in the early afternoon on Sadowski Field at Fort Hood.
Obama vowed after the shootings, "We're going to get to the bottom of exactly what happened."
White House senior adviser Dan Pfeiffer confirmed the Wednesday trip during an appearance Sunday on CBS' "Face the Nation." Obama also addressed the memorial service for an earlier massacre at Fort Hood, in 2009.
Then, Army Maj. Nidal Malik Hasan killed 13 people and wounded another 32. The former Army psychiatrist was convicted of premeditated murder, and a military jury recommended that Hasan be put to death.
CNN's Dana Ford contributed to this report.