BAE Systems, Northrup Grumman and United Airlines will receive $2 million each over six months to determine whether existing military technology can be used to counter the terrorist threat, said Charles McQueary, the Homeland Security Department's undersecretary for science and technology.

Existing defenses, such as infrared jammers that redirect heat-seeking rockets away from aircraft engines, already are used on military planes and Air Force One.

The companies, chosen from among 24 that sought the contracts, will develop and test prototypes and the government then will decide whether to choose one or more to more fully develop.

The Bush administration has been criticized by some lawmakers who say it has not taken the missile threat seriously enough. Hundreds, and perhaps thousands, of Soviet-style SA-7s — heat-seeking rockets that can hit low-flying aircraft within a range of three miles — are said to be available on the worldwide arms market.

Concerns about terrorists using lightweight rocket launchers to take down commercial airliners increased in November 2002 when terrorists fired two SA-7 missiles that narrowly missed an Israeli passenger jet after it took off from Mombasa, Kenya. Officials concluded that al-Qaida probably was behind the attack, which coincided with a bomb blast at a nearby hotel.

It's estimated that it would cost about $1 million per plane to install anti-missile systems. There are about 6,800 planes in the U.S. commercial fleet.

Airlines say the government should pay all costs for installing the devices.