The gunman was wounded in a shootout with airport police and taken into custody, authorities said. His condition was not immediately released.
The attack at the nation's third-busiest airport sent terrified travelers running for cover and, according to an airport official, disrupted 746 flights nationwide, either because they were held on the ground at LAX or not allowed to depart to other airports.
The slain employee was the first TSA officer killed in the line of duty in the 12-year history of the agency, which was founded in the aftermath of 9/11.
The FBI and Los Angeles Airport Police identified the suspected gunman as Paul Ciancia, 23, of Pennsville, N.J. He had apparently been living in Los Angeles.
A law enforcement official, speaking on condition of anonymity because they were not authorized to discuss the investigation publicly, said Ciancia was wearing fatigues and carrying a bag containing a handwritten note that said he wanted to kill TSA employees and "pigs."
Ciancia had at least five full 30-round magazines on him, said the official, who was briefed at LAX on the investigation. The official said Ciancia was shot in the mouth and leg by two airport police officers.
Early Friday afternoon, Ciancia's father in New Jersey had called authorities for help in finding his son after the young man sent one of his siblings a text message about committing suicide, Pennsville Police Chief Allen Cummings said.
The chief said he called Los Angeles police, which sent a patrol car to Ciancia's apartment. There, two roommates said that they had seen him Thursday and that he was fine, according to Cummings.
Cummings said that the Ciancias — owners of an auto body shop — are a "good family" and that his department had had no dealings with the son.
The attack began around 9:20 a.m. when the gunman pulled an assault-style rifle from a bag and began firing inside Terminal 3, Los Angeles Airport Police Chief Patrick Gannon said. The terminal serves such airlines as Virgin America, AirTran, Alaska Airlines, Horizon Air and JetBlue.
The gunman then went to the security screening area, where he fired more shots and went into the secure area of the terminal, Gannon said. Officers exchanged fire with him and seized him, Gannon said.
As gunfire rang out, panicked travelers dropped to the ground. Those who had made it past security ran out of the terminal and onto the tarmac or sought cover inside restaurants and lounges.
"We just hit the deck. Everybody in the line hit the floor and shots just continued," said Xavier Savant, who was waiting in the security line where the shooting took place. He described it as a "Bam! Bam! Bam!" burst of gunfire.
Savant said people bolted through the metal detectors and ran into the terminal.
"My whole thing was to get away from him," said Savant, an advertising creative director who was heading to New York with his family for a weekend trip.
As police searched for other possible shooters, they escorted travelers out of the airport, which continued operating but stopped some flights from taking off or landing.
Just a few weeks ago, airport police and the Los Angeles Police Department had jointly trained for a similar shooting scenario, according to Gannon, who said officers told him the drill was critical in preparing them for the real thing.
Across the U.S., aviation officials stopped LAX-bound flights from taking off from other airports, causing delays around the country. Some Los Angeles-bound flights that were already in the air were diverted to other airports.
At least three other TSA officers were wounded, said J. David Cox Sr., national president of the American Federation of Government Employees. Their conditions were not disclosed.
The officer who was killed was a behavioral detection officer, Cox said. Such officers are stationed throughout the airport, looking for suspicious behavior, he said.
Ben Rosen was sitting at the Starbucks eating oatmeal when he heard gunfire erupt and saw people running in all directions or crouching. He grabbed his phone and tried to lie as flat on the ground as he could.
Police showed up with guns drawn, shouting, "This is not a drill! Hands up!"
People put their hands up and then were led out of the terminal to the adjacent international terminal, Rosen said. As they were led out they saw broken glass from a window that looked as if it had been shot out. Rosen left his bag behind.
It was not the first shooting at LAX. On July 4, 2002, a limousine driver opened fire at the airport's El Al ticket counter, killing an airline employee and a person who was dropping off a friend at the terminal. Police killed the man.
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Associated Press writers Joan Lowy and Alicia Caldwell in Washington; Greg Risling, Christopher Weber, Alicia Chang, Alicia Rancilio, Gillian Flaccus and Michael R. Blood in Los Angeles; Josh Hoffner in Phoenix; and Michael Rubinkam in Pennsylvania contributed to this report.
