By Pete Williams, Andrew Blankstein, Erin McClam and Tracy Connor, NBC News

(NBC) - A gunman opened fire Friday inside a terminal at Los Angeles International Airport, killing one Transportation Security Administration worker, wounding as many as three others and sending terrified travelers diving for cover and fleeing onto the tarmac, authorities and witnesses said.

The gunman was in custody, multiple sources told NBC News.

The shooting started at about 9:20 a.m. local time at Terminal 3, at a security checkpoint where travelers show their IDs and boarding passes before they go through the metal detector, a TSA spokesman said.

Travelers who were waiting to snake through the security line suddenly had to abandon their suitcases and hit the ground, witnesses said. The shooting brought one of the nation's busiest airports to a standstill.

"We were just standing there in line and somebody started shooting," said Nick Pugh, a witness, who told NBC Los Angeles that he heard eight to 10 shots. "Everyone dropped to the floor and started crawling along the ground."

Federal officials, citing initial information, told NBC News that the suspect was a U.S. citizen in his early 20s.
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.

Brian Adamick, an accountant who was waiting for a flight to Chicago, said he saw a bunch of people running and one of them shouted, "There's shooting!"

He said he boarded an evacuation bus minutes later and saw a TSA agent get on, limping.

"His right ankle is injured and he gets on and sits in the seat next to me, and I said, ‘Are you OK?' He said, ‘I'm fine. I got shot. Don't worry, I've been shot before,'" Adamick told NBC News in a telephone interview.

"I said, let me take a look, and he pulls up the pant leg and there's a flesh wound — 3 to 4 inches — looks like it's straight out of Hollywood, maybe a drop of blood or two," he said.

Federal officials told NBC News that it was unclear whether the gunman was targeting the TSA or was trying to shoot his way through to get further into the airport.

The Federal Aviation Administration grounded flights at the airport, which serves more than 60 million fliers a year and where 1,500 flights take off and land every day. Passengers heading there departing flights were stuck in colossal traffic jams. Some abandoned their rides and rolled their suitcases the last distance to the airport.

Terminal 3 serves Virgin America, among other airlines.

Another witness told MSNBC that there was an initial round of shots, then a pause, then more shots. An airline worker told them, "Go out! Go out!" the witness said. People streamed down the stairs and onto the tarmac for safety, he said.

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

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

In 2002, an Egyptian national opened fire 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.

Kristen Welker of NBC News contributed to this report.