The attack sent terrified travelers running for cover and disrupted flights from coast to coast, authorities said.
A law enforcement official said the suspect, Paul Ciancia, 23, from Pennsville, N.J., was wearing fatigues and carrying a bag containing the hand-written note. The official was briefed at LAX on the investigation and requested anonymity because was he was not authorized to speak publicly.
A second law enforcement official confirmed the identity, also speaking on condition of anonymity because he was not authorized to discuss the investigation publicly.
Pennsville Chief Allen Cummings said Paul Ciancia's father called him early Friday afternoon saying another of his children had received a text message from the suspect "in reference to him taking his own life." Cummings said the elder Ciancia asked him for help in locating Paul, according to Cummings.
The chief said he called Los Angeles police, which sent a patrol car to Ciancia's apartment. There, two roommates said they had seen him Thursday and that he was fine.
Cummings said he told Ciancia's father that because of the son's age, he couldn't take a missing persons report. He said Ciancia's father owns an auto body shop. He says they are a "good family" and that his department had no dealings with the younger Paul Ciancia.
Los Angeles Airport Police Chief Patrick Gannon said that around 9:20 a.m., the gunman pulled what he described as an "assault rifle" from a bag and began firing inside Terminal 3. He 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 the gunman and apprehended him; police believe he was the only shooter, Gannon said.
"As you can imagine, a large amount of chaos took place in this entire incident," he said.
As gunshots rang out, panicked fliers dropped to the ground. Those who had made it past security fled 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 occurred. He described it as a "Bam! Bam! Bam!" burst of gunfire.
Savant said the shots subsided and people bolted through the metal detectors and ran into the terminal, eventually making their way out to the tarmac.
"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 shooters, they escorted travelers out of the airport. Aviation officials stopped flights destined LAX from taking off from other airports, causing delays across the country. Some flights also had to be diverted to other airports.
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.
The TSA officer shot at LAX airport was the first ever killed in the line of duty, union and TSA officials said. At least three other TSA officers were also injured, said J. David Cox Sr., national president of the American Federation of Government Employees.
The officer who was killed was a behavioral detection officer, Cox said. Those 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 people start running in all directions and others crouching on the ground. Rosen got on the ground and another passenger said: "Don't worry, we're safe."
Then, more gunshots erupted. He grabbed his phone and tried to lie as flat on the ground as he could.
Police showed up with their guns drawn, shouting, "This is not a drill! Hands up!"
Everyone put their hands up and then were led out of the airport terminal to the international terminal, Rosen said. As they were led out they saw broken glass from a window that looked like it'd been shot out. Rosen left his bag behind.
Six people were taken to the hospital, the Los Angeles Fire Department said. It's unclear whether the gunshot victims were among the group.
Associated Press writer Josh Hoffner in Phoenix and Michael Rubinkam in Pennsylvania contributed to this report.
Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.