A lone gunman has opened fire with an assault rifle at Los Angeles International Airport (LAX), killing a security agent, sparking chaos and causing widespread flight disruptions.
Panicked travellers scrambled to escape on Friday morning after the shooter, named as 23-year-old Paul Anthony Ciancia, pulled out the gun and shot his way through a security checkpoint before being stopped in an exchange of fire with police.
The victim was the first employee of the Transportation Security Administration (TSA) to be killed in the line of duty since the TSA was set up in the wake of the September 11, 2001 terror attacks.
Seven people were injured, but the lone gunman had lots more ammunition on him when he was arrested, said LA mayor Eric Garcetti.
"There were more than 100 more rounds that could have literally killed everybody in that terminal today," he said, praising airport police.
The gunman was reported to be in critical condition in hospital.
He opened fire at about 9.20am local time (3.20am Saturday AEDT) in a crowded terminal of LAX, the third-biggest US air transport hub which hundreds of Australians and New Zealanders pass through every day.
He "came into Terminal Three, pulled an assault rifle out of a bag and began to open fire," said LAX police chief Patrick Gannon.
"He proceeded up into the screening area... and continued shooting," he said.
TV footage showed people diving to the floor at the sound of gunfire and scrambling to escape the terminal.
Police chased the gunman to near a Burger King restaurant where they "engaged him in gunfire... and were able to successfully take him into custody," Gannon said.
Xavier Savant, who was waiting in the security line where the shooting occurred, described it as a "Bam! Bam! Bam!" burst of gunfire.
Ben Melling, an Australian living in LA, was waiting at the adjacent Tom Bradley Terminal for his wife to arrive from Sydney.
"The next thing sirens started going off and police just swarmed," Melling told AAP. "Guns were drawn," he said.
"People ran for cover.
"It was like I was in a Hollywood movie."
The TSA, which employs screeners and guards at airports, confirmed one of its employees had died.
An NBC television report suggested Ciancia, identified by the FBI as a New Jersey man resident in LA, had "strong anti-government views," and may have had some link to the TSA and targeted TSA agents as his victims.
A police chief in New Jersey said Ciancia's father called him on Friday saying another of his sons had received a text message from Ciancia in reference to him taking his own life.
Pennsville Chief Allen Cummings said he called LA police who sent a patrol car to Ciancia's apartment. Two roommates there said they had seen him on Thursday and he was fine.
A law enforcement official, who declined to be named, said Ciancia was carrying a bag containing a handwritten note that said he wanted to kill TSA employees and "pigs".
The official said it indicated Ciancia believed his constitutional rights were being violated by TSA searches and he was upset at former Department of Homeland Security Secretary Janet Napolitano. Officials told a press conference the gunman acted alone and they saw no further threat at the airport, a major gateway for flights to Asia, Australia and New Zealand.
Some 750 flights were disrupted after the Federal Aviation Administration (FAA) issued a national ground-stop.
In Washington, President Barack Obama was kept up to date on the shooting. "Obviously, we've been monitoring it and we're concerned about it," Obama said.