But the department isn't convinced that there is a cost-effective method to thwart a determined terrorist on the ground aiming at an aircraft overhead.

"We are in the process of determining if in fact there is a viable technology that could be deployed on commercial aircrafts," said Brian Roehrkasse, a spokesman. The department declined to identify the bidders, but Raytheon Co., Northrop Grumman Corp. and BAE Systems North America Inc. confirmed that they were pursuing the technology.

The department is not proceeding with any greater urgency because of the arrest of Hemant Lakhani, a Briton who was charged with selling a shoulder-fired missile to federal agents posing as terrorists, Roehrkasse said.

Because the arrest was the result of a sting operation, federal officials do not associate any new threat with it, he said. "The U.S. intelligence community does not have any specific intelligence that al Qaeda or any other terrorist organizations possess these weapons for use in a plot to shot down a commercial aircraft in the United States," he said.

The missile used in the sting was a dummy, U.S. Attorney Christopher J. Christie said.
Homeland Security plans to spend $2 million this year studying the proposals and could award contracts to develop prototypes next year, Roehrkasse said. The House and Senate Appropriations committees have approved separate plans to fund contracts valued at up to $60 million to deploy the antimissile technology. The agency does not know when the technology could be deployed, Roehrkasse said.

Sen. Charles E. Schumer (D-N.Y.) has called the administration's approach to the issue "all too halting, slow and incomplete." Arguments that the technology is not ready for commercial aircraft are faulty, Schumer said last week in a letter to President Bush.

The military already uses this technology, and "this year alone, it has helped foil at lease three shoulder-fired-missile attacks on military cargo planes in Iraq," he said. Schumer estimated that it would cost $7 billion to $10 billion to outfit civilian airplanes with the technology.

The Pentagon has long used high-tech "countermeasures" on military planes to detect and destroy shoulder-fired missiles and other heat-seeking surface-to-air weapons. Several Army helicopters are outfitted with a laser-based system developed by BAE. When the helicopter's sensors detect an incoming missile, a laser tracks it and emits enough energy to confuse it, sending it off course.

But as the defensive system becomes more sophisticated so do the missiles, said Steven J. Zaloga, a weapons expert at Teal Group Corp., a defense research firm. Detecting "the signature of the missile being fired is not that simple, especially around crowded urban environments, like around airports," he said.

Adapting military technology for commercial use could be difficult. Some combat planes fire flares as they take off to confuse incoming missiles, a maneuver that would be ill-suited to commercial aircraft, Zaloga said. "If they take off from National Airport, you can't pop off flares. . . . They are a fire hazard," he said. "What's good in a military environment is not necessarily good in a civilian environment."

Many advanced missile defense systems require routine and expensive maintenance, Zaloga said. That would be inconvenient for a commercial plane that is flown regularly, he said.

Northrop Grumman recently upgraded its laser-based technology that jams the guidance system of incoming missiles, said Robert DelBoca, Northrop's vice president of infrared countermeasure systems. With a large order, Northrop Grumman could deploy an adapted version of the system for $1 million a plane, DelBoca said.

Raytheon is developing a system that would detect an incoming missile and unload hundreds of three-inch metal wafers that oxidize, or rust, emitting heat signals that confuse the weapon. The wafers would dissipate before reaching the ground, said Alan Howell, director of strategy and business development at Raytheon's Electronic Warfare Systems unit.

Staff writer Sara Kehaulani Goo contributed to this report.