A U.S. Patriot missile battery mistakenly shot down a British Royal Air Force Tornado GR4 fighter plane near the Kuwaiti border yesterday as the aircraft returned to its base after striking Republican Guard forces outside Baghdad, U.S. and British defense officials said.

The Royal Air Force confirmed that both of the aircraft's crew members were dead but did not release their names, pending notification of next of kin. They were the first "friendly fire" casualties of the war in Iraq. Two helicopter mishaps have also claimed the lives of 14 other British servicemen and five Americans.

Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, said on ABC's "This Week" that electronic procedures for identifying and differentiating friendly and enemy aircraft "broke down somewhere."

Those procedures, he noted, are meant to "ensure safe passage" for friendly aircraft and to detect Iraqi planes or drones that might be carrying "chemical or biological weapons on them heading into our large force concentrations in Kuwait." The U.S. Central Command is investigating the incident.

Defense Secretary Donald H. Rumsfeld, appearing on NBC's "Meet the Press," said that the likely cause of the accident was either a failure by the aircraft to emit a radio signal identifying it as a friendly aircraft or a failure of the Patriot missile battery to properly "interpret" such a signal.

British Maj. Gen. Peter Wall, briefing reporters at the Central Command headquarters in Doha, Qatar, said "there is clear evidence to suggest that the U.S. Patriot missile battery shot down an RAF Tornado."

"We need to establish exactly how this happened so we can take steps to minimize the risks in the future," Wall said. "But we have checked our current procedures, and we are satisfied."

Patriot missile batteries, much improved from the rudimentary Patriots fired at Iraqi Scud missiles during the 1991 Persian Gulf War, have been stationed in Kuwait and throughout the Middle East to intercept Scuds and protect coalition forces from attack by Iraqi aircraft or drones.

U.S. officials have reported for months on Iraq's efforts to build remote-controlled drones and equip them with sprayers that could be used to attack vulnerable base camps with chemical or biological weapons.

Patriot batteries in Kuwait shot down two Iraqi missiles fired at U.S. forces in Kuwait last Thursday, the first day of the war. It was unclear whether the Iraqi missiles were Scuds or shorter-range Ababil-100s or Al Samoud-2s, but there is no evidence they carried chemical or biological weapons.

The Patriot battery that shot down the British plane was based outside Camp New Jersey in Kuwait. It was part of the 5th Battalion, 52nd Air Defense Artillery Regimentof the 11th Air Defense Artillery Brigade, part of the U.S. Army's 5th Corps.

Two upgraded versions of the Patriot have been stationed in Kuwait, and both were fired last week at the Iraqi missiles. It was not clear which type was launched at the British aircraft. The latest Patriot, known as the Patriot Advanced Capability-3 (PAC-3), is a "hit-to-kill" system that senses the incoming trajectory of a missile or aircraft and guides the Patriot interceptor directly into it.

Earlier versions of the Patriot use blast warheads designed to detonate close to an incoming missile or aircraft. U.S. officials initially claimed during the 1991 Gulf War that the Patriots had been highly accurate in intercepting Scuds aimed at Israel and Saudi Arabia, but further analysis showed their blast warheads failed more often than not to intercept the Iraqi missiles.