LOS ANGELES (REUTERS) - The Los Angeles International Airport terminal where a gunman opened fire on Friday morning, killing an unarmed federal airport security officer and wounding others, re-opened to travellers the day after the shooting as authorities probed the motive behind the attack.

Authorities have identified the suspected shooter as Paul Anthony Ciancia, 23, and they said he was shot and wounded by police in an exchange of gunfire at Terminal 3 at one of the world's busiest airports.

Authorities said the gunman fatally shot one TSA officer, identified as Mr Gerardo Hernandez, 39, who was the first from the agency to be killed in the line of duty.

Mr Hernandez was at a document verification checkpoint when he was shot, said a TSA official who spoke on condition of anonymity. Two other TSA officers were wounded, the official said.

The gunman, a United States (US) citizen who appeared to be acting alone, pushed through the screening gates and ran deep into the passenger boarding area at Terminal 3, before law enforcement officers caught up with him in a food court, said Mr Patrick Gannon, chief of the Los Angeles Airport Police.

The FBI on Saturday had not provided the total number of people shot in the attack. Paramedics took five who were wounded at the scene of the shooting to area hospitals, Los Angeles Fire Department officials said.

The Los Angeles Times reported that among the wounded was teacher Brian Ludmer, 29, who was shot in the leg. Ludmer teaches high school in the Los Angeles suburb of Calabasas.

Ronald Reagan UCLA Medical Center received three men wounded in the shootings, with one individual listed in critical condition on Saturday and one in fair condition, while the third was treated and released on Friday, hospital spokesman Mark Wheeler said in a statement.

The hospital did not release identities of its patients, but the Los Angeles Times, citing a law enforcement source, said Ciancia, with leg and head wounds, was in critical condition at the hospital, which hampered police efforts to interview him.

The airport on Saturday began allowing travellers who abandoned luggage and other property as they ran to escape the gunfire to collect their belongings at ticket counters.

The area of Terminal 3 beyond metal detectors where travellers board flights was re-opened to passengers at mid-day on Saturday, said Ms Gina Marie Lindsey, executive director of Los Angeles World Airports. The terminal zone where passengers pick up boarding passes had earlier resumed operations.

"Barely 24 hours after this opened, Terminal 3 was partially opened. Less than 30 hours after this incident this airport is going to be fully functioning. That is a remarkable achievement," said Los Angeles City Councilman Mike Bonin.

TRIBUTE TO OFFICER

Los Angeles police officers will be wearing black mourning bands in memory of Mr Hernandez, chief Charlie Beck of Los Angeles Police Department said on Twitter.

Meanwhile, the airport said its 100-foot pylons would light the night blue through Sunday to honor Mr Hernandez.

"RIP," the post said.

Late on Friday, FBI agents obtained a search warrant and combed through Ciancia's home in the suburban Sun Valley section of Los Angeles, FBI spokeswoman Ari Dekofksy said.

Mr John Mincey, Ciancia's former roommate from the Los Feliz neighbourhood of Los Angeles, told local television station KABC that Ciancia never displayed hatred or ties to "any hate group, or anything like that."

Armed with an assault rifle, the shooter touched off panic and chaos. Hundreds of travellers ran for safety or frantically dove for cover behind luggage, as loud alarms blared through the terminal.

Traveller Lauren Stephens, 47, said she had just put her luggage on the scale at the ticket counter in Terminal 3 when she heard gunshots. "Somebody just yelled 'Run!' at the top of their lungs. ... I just left my bag and I just ran like hell.

Everybody ran." The investigation into the attack will examine the shooting itself as well as the gunman's background and motivation, Special Agent David Bowdich of the FBI said on Friday. "Our goal is to do a true scrub on the individual to find out what was the tipping point for this person," he said.

Los Angeles Mayor Eric Garcetti has asked that flags on city buildings be flown at half-staff, local media said.

DISTURBING TEXT MESSAGE

In New Jersey, police and FBI agents descended on Ciancia's family's home in Pennsville Township.

Pennsville Police Chief Allen Cummings said he had been contacted by Ciancia's father before the shooting, prompted by a worrisome text message from the young man to his brother.

The police chief declined to reveal more about the content of the text message but said that family members told investigators they had no previous indications that Ciancia, who moved to California about 18 months ago, was troubled.

Neighbours who live across the street from the Ciancia family said the father, also named Paul, runs an auto body shop in the town.

"I believe he worked for his father," said one neighbour, Jennifer Pagan, of the younger Paul.

Her husband, Orlando Pagan, said the elder Ciancia had made several friendly gestures since they had moved into their house 10 years ago. When Hurricane Sandy hit New Jersey last year, "he asked if we wanted to take our personal vehicle and put it on his property." The Ciancia property is slightly higher.