The two incidents a half-hour apart on the busy Labor Day weekend appeared to be unrelated, said FBI spokeswoman Cathy Viray. Several people suffered minor injuries.
The scare at the international terminal apparently came when a flashlight battery exploded as it was being screened by a Transportation Security Administration worker, Viray said.
"When it was going through screening, the TSA individual touched it and at that point it exploded," she said.
The TSA worker suffered minor injuries to his hands, Viray said. Airport spokesman Tom Winfrey said the small explosion "slightly injured several people." The passenger whose bag was being screened at the time was being questioned.
About half an hour earlier, a passenger bypassed security at United Airlines' Terminal 8, prompting the evacuation of terminals 6, 7 and 8, Winfrey said. Authorities cleared the terminals, which are connected, in order to re-screen passengers, he said.
About 30 departing United flights were listed as delayed, and 17 of the airline's inbound flights were listed as canceled or delayed.
Traffic was diverted from the airport and hundreds of people could be seen standing outside the airport.
The shutdown Saturday followed a bomb scare Friday at Ontario International Airport that prompted the evacuation of 1,000 people for two hours. The suspicious bag that caused it turned out to contain only cosmetics, officials said.
Authorities are especially wary of terrorism at Los Angeles International Airport, among the world's busiest. It has twice been targeted for attacks - a foiled bomb plot planned for around New Year's Day 2000, and a shooting at a check-in counter that left three dead on July 4, 2002.