With the U.S. Army leading the investigation — not civilian agencies — the public may never know all the details of this week's deadly shootings at Fort Hood, Texas. But investigators are likely looking to answer these questions:
What Triggered the Rampage?
Military authorities and law enforcement officials say it's too early to know why Spc. Ivan A. Lopez opened fire Wednesday, killing three fellow service members and wounding 16 others before committing suicide.
The base commander, Lt. Gen. Mark Milley, has said several times that nothing has been ruled out but that it appeared the incident had nothing to do with radicalism or terrorism.
Milley and other military officials say Lopez was being treated for anxiety and depression and was being evaluated for possible post-traumatic stress disorder. Although he never saw combat in the Army, Lopez also "self-reported" a severe head trauma.
But it's not at all clear that any of that had anything to do with Wednesday's shootings. Officials say it's probable that the tragedy simply sprang from an argument with another service member -- with whom and over what remain unknown.
Who Was Shot?
A couple of the victims' identities have emerged in news reports — a sergeant from Illinois and a major from New York state — but officially authorities will say only that all of them were service members.
Seven were treated on base at Carl R. Darnall Army Medical Center, where three of them remained Thursday night. Base officials wouldn't disclose their conditions.
The nine others, described as the most seriously injured, were treated in intensive care at Scott & White Memorial Hospital in nearby Temple, where doctors described gunshot wounds to people's arms, legs, abdomens, chests and necks. Four of them remained in the hospital Thursday night, three in serious condition and one in good condition.
Who Fired Shots?
Military officials haven't said. Milley said Lopez was confronted by a female military officer, who pulled her weapon and "engaged," whereupon Lopez shot himself in the head. He didn't say what he meant by "engaged."
Lopez is blamed for having shot at least 19 other people plus himself with a .45-caliber Smith & Wesson semiautomatic pistol. Smith & Wesson says that, depending on the specific model, its .45s can hold seven to 11 rounds. That means Lopez stopped to reload, or that one or more other people — perhaps the MP — also fired shots, or both.
Why Was He Depressed?
Authorities haven't said what led to Lopez seeking treatment for depression, but Lopez's mother died in November.
The gunman was upset at the process for getting leave to attend her funeral, according to Glidden Lopez Torres, who is not related to the gunman but identified himself as a family friend.
Lopez was granted only a short leave — 24 hours, later extended to two days — to go to her funeral, which was delayed for nearly a week so he could make it, Torres told the Associated Press.
Yaritza Castro, who grew up with Lopez and now lives in Miami, told the AP: "That was a very frustrating time for him."
Lopez was also transferred to a transportation company at Fort Hood just two months ago after having spent 15 years in National Guard and Army infantry regiments. But no indication has yet emerged that he was unhappy about the change of assignment.
What About the Gun?
Why Lopez had a private weapon on base hasn't been explained. He lived off the base, so base officials wouldn't have known he even had one, Army Secretary John McHugh told the Senate Armed Services Committee on Thursday.
All service members must register private firearms with the base — but Lopez didn't do that.
Investigators told NBC News they have no reason to suspect that Lopez bought the gun — from the same gun store where Maj. Nidal Hasan bought his before killing 13 people at Fort Hood in 2009 — intending to shoot anybody at the base. The explanation could be as simple as that he was replacing one that had been stolen.
Could He Have Been Stopped?
Defense Secretary Chuck Hagel promised to get to the bottom of that.
It's clear that "something's not working," Hagel said Wednesday, even though U.S. bases have significantly beefed up security since the Sept. 11, 2001, terrorist attacks — and especially since the 2009 shooting.
All personnel are supposed to carry their registered private firearms through a specific gate, where they're subject to inspection. Lopez apparently didn't do that Wednesday.
The base conducts random compliance checks, the Defense Department said.
But they're of limited use when dealing with a facility with more than 40,000 active-duty service members, almost 20,000 civilian and contract employees, and tens of thousands of military retirees.