The evacuations at three interconnected domestic terminals came on Labour Day weekend, one of the busiest travel holidays of the year, after a person bypassed security by walking up an exit staircase into the passenger area, local officials said on Saturday.
The authorities were unable to locate the person who triggered the alarm but all passengers in the terminals were evacuated and screened again.
A separate incident about 30 minutes later shut down the airport's busy international terminal when a plastic flashlight exploded in the bag of a Japanese tourist about to board a Korean Air flight for Tokyo, Los Angeles Mayor Jim Hahn told a news conference.
Police chief Bill Bratton added the batteries in the flashlight apparently malfunctioned, causing a small explosion.
Eight workers, including five with the Transportation Security Administration and three baggage handlers were taken to the hospital for treatment for minor injuries, Hahn said.
"At this time we don't believe the explosion involved any intentional explosion or explosive device," Hahn said. "But that remains under investigation."
The domestic terminals were evacuated after 7.30 am (1430 GMT) and authorities later closed off access to the central terminal and blocked off surrounding roads leading into the nation's third-busiest airport.
Flights have been delayed all day
because of the security scare
Officials reopened the domestic terminals and roads shortly before noon.
A spokeswoman for United Airlines, which has a large presence in the affected terminals, said arriving and departing flights would be delayed all day.
The airport, which has eight domestic terminals and one dedicated to international flights, was the third-busiest for passenger traffic in the United States in 2003 and the fifth-busiest in the world, according to the Airports Council International.