LOS ANGELES (AP) -- American Airlines Flight 11 bound for Los Angeles from Boston was one of the planes that crashed into the World Trade Center Tuesday morning, airline officials said.
The airlines said the flight was hijacked after takeoff. The number of passengers on the plane could not be immediately determined.
At military installations across Southern California, including Vandenberg and Edwards Air Force bases, officials did not immediately step up security, pending word from the Pentagon.
"We have not changed our force protection condition," said Sgt. Rebecca Bonilla, a Vandenberg spokeswoman.
Meanwhile, Los Angeles International Airport was under tight security.
Airport spokeswoman Gail Gaddi said departing flights were not being allowed to take off by order of the Federal Aviation Administration. Arrivals were being allowed to land.
Ontario International Airport was also taking heightened security precautions.
"The airport has initiated high security procedures following the FAA directive," Ontario police Lt. Robert Keller said.
He said there had been no incidents, but the airport was closing down and Ontario police were working with airport authorities to shut operations down.
Several messages left for a Department of Airports spokeswoman at Ontario International Airport were not immediately returned.
At LAX, police were not allowing people to unload baggage to board flights at the American Airlines terminal.
At least 20 Los Angeles police officers were at the terminal.
One officer Lt. Howard Whitehead said, "All we know is what we've heard from radio and television. We're not allowing any unattended vehicles near the terminals."
Vehicle traffic was extremely light at the terminal area and the parking lot near the American terminal and the large international terminal were about only about one-third full.
Many people with canceled flights were at the American counter seeking information. Others were carrying and pulling their bags, looking for taxis and other ways to leave after their flights were canceled.
"What can you say? It's the only thing the government could do is to shut down the airports," said Don Kahmann of Ventura, whose flight to Kalamazoo, Mich., with his wife, Madeline, was canceled by American Airlines.
"They haven't told us anything at all," said Madeline Kahmann as the couple waited with about 100 other people to talk with airline counter agents.