A gunman walked into Terminal 3 at Los Angeles International Airport Friday morning and opened fire at a security checkpoint, killing a Transportation Security Administration officer and wounding at least three other people, law enforcement sources tell CBS News.

The gunman, identified by law-enforcement officials as 23-year-old Paul Anthony Ciancia of New Jersey, was shot by law enforcement and taken into custody in critical condition. They believe he had anti-government views based on the material he was carrying, the officials said.

A law-enforcement official said Ciancia was wearing fatigues and carrying a bag containing a hand-written note that said he “wanted to kill TSA and pigs,” the official said, who was briefed at LAX on the investigation and requested anonymity.

“He went past the screeners and back into the airport itself,” said Los Angeles Airport Police Chief Patrick Gannon at a press conference.

Police then “engaged him with gunfire” and took him into custody. He added that officials did “security sweeps” through the airport.

Gannon said there was only one shooter.

UCLA Medical Center said it had one patient in critical condition and two in fair condition. One of the TSA workers was shot in the leg, said Nico Melendez, a spokesman for the Transportation Security Administration.

“We are sickened by reports of today’s shooting at the Los Angeles International Airport,” TSA Union Leader J. David Cox Sr. said. “Our sincerest thoughts and prayers go out to the passengers and Transportation Security Officers killed or injured in this heinous act. Thank you to all of our brave TSOs who put their lives on the line every day to keep the flying public safe. AFGE is monitoring the situation along with TSA management.”

Los Angeles Police Department anti-terrorism officials were on the scene, but sources said they don’t believe the suspect was tied to any known terrorist groups.

An eyewitness, Leon Saryan, told ABC News Radio that a “fairly young” gunman with a rifle came down a corridor in Terminal 3 and shot perhaps 20 times.

A TSA agent holding Saryan’s shoes was shot and wounded, he told ABC.

Rescue vehicles outside Los Angeles International Airport on Nov. 1, 2013

Saryan said the gunman asked him, “Are you TSA?”

“He saw me. He looked at me with a quizzical look and said TSA? And I just shook my head. So he moved on,” Saryan said.

LAPD Assistant Chief Earl Paysinger says the suspect, who was armed with a semi-automatic rifle, was apparently injured in an exchange of gunfire.

Police, who shut down streets around the airport, said a possible second suspect was also taken into custody outside Terminal 2.

Video from NBC Los Angeles showed emergency vehicles outside one terminal. The station reported several people, including what appeared to be a TSA agent, had been loaded into ambulances.

WATCH LIVE COVERAGE: TSA agent shot, major police response

NBC Los Angeles reported the first reports of gunfire came at about 12:20 p.m. ET. Half an hour later, flights were still taking off and landing, the station reported.

The Federal Aviation Administration later grounded flights, the Los Angeles Police Department confirmed on Twitter.

“We were just standing there in line and somebody started shooting,” Nick Pugh told NBC Los Angeles. “Everyone dropped to the floor and started crawling along the ground, abandoning their suitcases.”

Alex Neumann was at a food court, waiting to travel to Miami, when the incident unfolded.

“People were running and people getting knocked down. There was luggage everywhere,” Neumann told CNN. “Mayhem is the best I can describe it.”

Witnesses said the shooting happened at Terminal 3, which serves Allegiant Air, Frontier, JetBlue, Spirit, Virgin America and Virgin Australia.

Bill Reiter, a writer for FoxSports.com, was at the scene and sent out a tweet: “When gunfire broke out there was a stampede people, all of us hiding under seats we didn’t fit under, we burst through the door to outside.”

Passengers were being herded along the sidewalk outside a terminal at LAX on Friday. Photo: KTLA-TV

Tory Belleci of the television show “Mythbusters” tweeted: “Heard gun shots then everyone starting running for the door. Not sure if anyone was hurt. #LAX.”

TV footage showed dozens of officers swarming the airport. Images also showed a law enforcement officer being treated by paramedics. He appeared to be alert. Another officer had a bloody hand.

Amateur video showed evacuated passengers stranded on the tarmac and inside the airport after police shut down the airport.

President Barack Obama was briefed on the shooting, the White House said in a statement.

“The President has been briefed about the shooting at LAX. We will continue stay in touch with our federal and local partners. The LAPD is leading the response and investigation.  We urge citizens to listen to the authorities and follow directions from the first responders on site. The President will continue to receive briefings throughout the day.”

In 2002, an Egyptian national opened fired at the El Al ticket counter at LAX, killing two Israelis before he was shot dead. Authorities ruled it a terrorist incident, even though the shooter was not tied to a known group.

This is a breaking news story. Check back for updates.

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