Authorities say the Washington Navy Yard shooter Aaron Alexis had a history of psychiatric problems and his father told police previously that his son had anger management problems ever since helping people in New York during 9/11. NBC's Pete Williams reports.

Many questions about Monday's shooting deaths of 12 people at the Washington Navy Yard remain unanswered and with the main suspect among the dead, some of the specifics may never be nailed down.

After using a fingerprint to identify him, authorities said the gunman was Aaron Alexis, 34, a former naval petty officer. Initially authorities said they were actively seeking a possible second suspect: a 50-year-old black man with a rifle, wearing an olive drab military uniform, but then late Monday they said they were no longer looking for that individual. A third man who was initially a person of interest was eventually cleared.

Investigators have revealed little of what they've found out, but here's what they're likely to be focusing on:

Why did he do it?

Slideshow: Navy Yard shooting / A gunman opened fire Monday at the Washington Navy Yard in Washington D.C., killing at least 12 people, authorities said. Launch slideshow

After having been discharged from the Navy, Alexis served as a naval reservist and had been working as a civilian subcontractor for a division of Hewlett-Packard that was upgrading equipment used on the Navy/Marine Corps intranet, the company confirmed Monday. Investigators said he may have recently lost that job, giving him a grudge to nurse.

Under a new support contract, all HP hardware at the Navy Yard was relocated to Denver last month — leaving few if any support jobs needed in Washington. It wasn't known whether Alexis' job was one that was moved to Colorado — and if so, whether he declined to move or was let go as part of the reorganization. HP wouldn't say Monday whether Alexis was still on its payroll.

Or maybe it was something else.

Court records show that Alexis was arrested in 2004 in Seattle for allegedly shooting a vehicle in what he told police was a "black-out fueled by anger." According to the original incident report, Alexis' father, who lived in New York, told police his son had "anger management problems" probably exacerbated by the stress of having been "an active participant in rescue attempts of September 11th, 2001."

Records do show that Alexis had a residence in New York throughout 2001.

How did he get into the Navy Yard building?

Valerie Parlave, assistant director in charge of the FBI's Washington field office, said Alexis had "legitimate" access to the complex.

Alexis had worked for a subcontractor for Hewlett Packard Enterprises called "The Experts," which does work in the Navy Yard complex, according to a statement from the company. He appeared to have an access card that would have allowed him onto the Navy Yard and other military installations, the company’s CEO, Thomas Hoshko, told The Washington Post.

Where did he get his guns?

The Washington Navy Yard added a tragic chapter to its centuries-long history Monday, as authorities say Aaron Alexis, a 34-year old former Naval petty officer, opened fire, leaving 12 people dead. Alexis was killed as well. NBC's Natalie Morales reports.

A shotgun used in the shootings was bought at a Virginia gun store within the last few weeks, multiple federal law enforcement officials told NBC News. Authorities believe he brought the shotgun to the Navy Yard with him and then acquired two other weapons — an AR-15-style assault rifle and a handgun — as he began the spree, taking one of them from a police officer he shot.

Although Alexis was arrested in 2010 on misdemeanor charges in Fort Worth, Texas, for accidental discharge of a weapon, those charges were dropped and wouldn't have prevented him from passing a background check. And indeed, Oui Stuhamtewakul of Fort Worth, Texas, who said he was Alexis' best friend and a former roommate, said Alexis did own a gun and had a concealed-weapons permit.

Did he target his victims?

Witnesses say the gunman never said a word during his rampage — giving little insight into his motivation. Washington Police Chief Cathy Lanier said, however, that had the gunman not been killed, "there's no question he would have kept shooting."

Did friends or family have any clues?

NBC News Criminal Justice Analyst Bill Bratton tells TODAY that the motivations and history of Navy Yard shooting suspect Aaron Alexis remind him of LAPD officer Christopher Dorner, who was charged in shooting attacks on police officers.

Friends and relatives of Alexis told reporters they were mystified, saying he was a Buddhist who was friendly and outgoing. In the words of one friend, Alexis "could not be the shooter."

But the Seattle police report indicates that as early as 2004, his father was concerned about Alexis's "anger management problems" and suggested that his son may have suffered from post-traumatic stress disorder because of his purported experiences on 9/11.

How did he die?

Police and the FBI said the gunman engaged in "multiple" firefights with them, but police maintained Monday that it still wasn't clear exactly how he died.

Robert Dembo, Michael Isikoff and Courtney Kube of NBC News contributed to this report.

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