While many details remain unclear about the shooting rampage that killed a Transportation Security Administration worker Friday at Los Angeles International Airport, here are some of the questions investigators are trying to answer:
Who is he?
Authorities identified the gunman as Paul Anthony Ciancia, 23, of Los Angeles, who was shot in the chest by law enforcement and hospitalized in critical condition.
Ciancia grew up in Pennsville, N.J., but was recently living in California, according to his father, also named Paul, who owns an auto repair shop in Pennsville. He said he didn't know what his son did for a living.
NBC Philadelphia reported that Ciancia attended Salesianum High School, a Catholic school in Wilmington, Del. It couldn't immediately be learned whether he'd gone to college.
How did he do it?
He simply entered through the exit.
Officials told NBC News that the shooting started at about 9:20 a.m. (12:20 p.m. ET) after the gunman walked through a lane reserved for passengers leaving the secure area of the airport.
Witnesses said he walked to the security checkpoint area. Then he pulled a .223-caliber semiautomatic assault-style rifle out of a bag and opened fire at one of the checkpoints.
Still firing, the gunman made it past the screeners "and continued shooting into the airport itself," airport Police Chief Patrick Gannon said.
At a cluster of restaurants in the rear of the terminal, near the waiting area for some of the gates, airport police shot him several times in the chest, law enforcement sources said.
Los Angeles Mayor Eric Garcetti said the suspect had more than 100 rounds remaining and "could have literally killed everybody in that terminal today."
Why did he do it?
It's too early to have a clear picture, and the FBI said investigators "have neither ruled out terrorism, nor ruled it in." But they appear to be tracking two possibly related angles.
Law enforcement sources told NBC News that the gunman was carrying anti-government literature outlining an alleged conspiracy to create a single global government. Sources said it expressed animus toward racial minorities and included specific references to the TSA.
According to a law enforcement official quoted by The Associated Press, a note with the literature said the gunman "wanted to kill TSA and pigs."
Witness accounts agreed.
Leon Saryan, a traveler in Terminal 3, told MSNBC that when the gunman approached him, "all he said was, 'TSA?' Just like that." Stephanie Rosemeyer, another witness, told the Los Angeles Times that when she took a step toward the gunman, he shouted an expletive directed at the TSA.
Why the gunman was so angry at the TSA remained a mystery. Tim Kauffman, a spokesman for American Federation of Government Employees, the TSA screeners' union, said Ciancia didn't work for the agency.
Meanwhile, Ciancia's teenage brother told Pennsville police that he had received a text from Ciancia early Friday saying he was "not going to be alive much longer."
Pennsville Police Chief Allen Cummings told NBC News that he asked Los Angeles police to conduct a "well-being check" at Ciancia's apartment. He wasn't home, and roommates told them everything was fine, Cummings said.
That was at about 10 a.m. — more than a half-hour after the shooting had started — NBC Los Angeles reported.
Ciancia's father told NBC Los Angeles that when he last spoke to his son a week ago, Ciancia complained that the economy was depressed.
How did he get the gun?
Authorities aren't yet saying, but however he acquired it, he was carrying it illegally — loaded weapons are forbidden in airports unless you're a law enforcement agent with a special reason to have one. Even the TSA agents at checkpoints are unarmed.
But did the gunman have the weapon legally otherwise? Probably not. Authorities described the weapon as a .223-caliber AR-15 semiautomatic rifle. "AR-15" is a specific model made by Colt, but other gun companies manufacture knock-offs, which are often generically referred to as AR-15s. Transporting or importing most of those is banned in California.
There are exceptions depending on who manufactured the weapon or whether the owner has made several complicated modifications that can make it legal. Outside those exceptions, an AR-15 is generally defined as an "assault weapon," the transportation or importing of which is illegal in California.
Gannon, the airport police chief, didn't specify the manufacturer, so theoretically the AR-15 used Friday could have been legal. But he also expressly called it an "assault rifle" — and transporting those is a felony.
Was the security response appropriate?
The Los Angeles airport used to have armed police officers protecting TSA checkpoints, but that changed several months ago, when they were redeployed to roving patrols, the Los Angeles Times reported.
Could those reassignments have slowed the law enforcement officers' response Friday? Not according to Gannon, who asserted that the changes actually improved security.
"Our officers did what they were supposed to do and performed heroically," he said.
Bill Bratton, former chief of police in both Los Angeles and New York, agreed that the security response was proper.
"Based on the quick response of the officers and ability to take him down before he was able to do more injury, I think it worked," Bratton told NBC News. "It was a security breach, certainly, but one that was contained quickly."