(ABC News) -- Police are investigating possible motives for the shooting at Los Angeles International Airport after a gunman brazenly entered a terminal at Los Angeles International Airport and opened fire.
The shooter, identified by the FBI as 23-year-old Paul Ciancia, pulled an assault rifle from a bag and blasted his way past airport screeners, killing a TSA agent and wounding several others, said Los Angeles World Airports Police Chief Patrick Gannon.
The shooting began around 9:20 a.m. PST on Friday at LAX's usually crowded Terminal 3, and sent hundreds of passengers streaming out of the terminal, with many fleeing onto the airport runway. Dozens of flights to and from the airport were delayed or cancelled as a "tactical alert" was triggered for the Los Angeles Police Department.
Witnesses described a chaotic scene, as many ducked for cover inside bathroom stalls or dropped to the floor upon officers' commands.
"I heard gunshots, and a few seconds later, I saw him coming up the elevator and walked by about 10 feet away from me with his gun pointed," Andrea Trujillo, who was at the terminal when shots first rang out, told ABC News.
Jose Martes, who was at the airport with his wife, Miriam Rodrigez, awaiting a flight to Norman, Okla., said he didn't realize what was happening at first.
"There was a loud bang. At first we looked at each other and everyone in line. We're like, OK, nothing unusual," he told ABC News. "We thought it was something else, but from, like a couple of seconds from that, that's when we heard just straight shots going down. That's when we all fell to the ground and we knew that this was not a test."
Authorities said Ciancia was able to make it all the way to the back of the terminal, near the departure gate, before he was shot down by officers and taken into custody, according to Mayor Eric Garcetti.
David Bowdich of the FBI said Ciancia is receiving medical attention at a hospital.
As law enforcement officials work to uncover Ciancia's motivations, a note found at the scene that indicated Ciancia's anti-government sentiments and suggested that he expected to die in the airport shootout.
The note found at the scene ended with the letters "NWO," according to law enforcement sources, which is believed to stand for "New World Order." The note also specifically mentioned anger and frustration targeted toward the TSA.
Ciancia's family was also concerned for his well-being. Ciancia's father contacted police in Pennsville, N.J., on Friday with the concern that his son may be suicidal, Chief Allen Cummings of the Pennsville Police Department told ABC News.
"Their younger child got a text message from Paul stating that there were some comments in there about his wellbeing, and he wanted to possibly take his own life," Cummings said.
Cummings said he called the LAPD today and asked for officers to "try to get a wellbeing check" on Ciancia.
Officers went to Ciancia's apartment in Los Angeles before the shooting and talked to his roommates, Cummings said.
"He was a really nice guy," one of his roommates told ABC News. "A little introverted, but nothing I would ever, ever expect him to do."
Ciancia is believed to be from Pennsville, N.J., however authorities said he has a residence in Los Angeles. Overnight, officials searched both his east and west coast residences.
Witnesses described Ciancia as a short, young, white man, carrying a long-rifle.
One of the shooting victims was a TSA officer who died of his wounds. The officer was a behavioral detection officer, assigned to identify passengers acting strangely, said J. David Cox, president of the AFGE, the officers' union.
The TSA confirmed the identity of the officer killed as Gerardo I. Hernandez, 39, who has been with the TSA since 2010.
The officer is the first TSA employee to be killed in the line of duty.
"It's a sad occasion," Cox said.
Six people were injured, including three TSA officers. Five were taken to nearby hospitals.
"One arrived in critical condition and two are listed in fair condition," a spokesman from Ronald Reagan UCLA Medical Center said in a statement.
An emergency physician said the patients suffered bullet wounds and other injuries. Two other patients were taken to other hospitals.
"No words can explain the horror that we experienced," TSA Administrator John Pistole said in a statement.