Perimeter and air patrols are being beefed up at Los Angeles International as defense against possible shoulder-fired missile attacks on commercial aircraft, a top security official said.

Security officials are undergoing training specific for that threat, and additional helicopter patrols are being deployed, John Miller, head of the Los Angeles Police Department's counterterrorism bureau, told Airports Friday. While air patrols would not be able to counter a launched missile strike, they could give officials an important vantage point over the adjacent waterfront.

And, Miller added, if an attack were launched several miles away, an air patrol could trace the contrails back to the assailants.

"This is something terrorists are already trying," Miller said.

Miller noted that shoulder-fired missiles have been used against commercial flights in recent years, most recently against a cargo plane at Baghdad International. About 20,000 shoulder-fired missiles are on the black market, and no one knows how many could have been smuggled into the U.S., he said.