The U.S. soldiers who mistakenly shot down a British warplane last year during the invasion of Iraq thought the plane was a missile heading toward them, according to official U.S. and British investigations.

U.S. military authorities absolved the soldiers of negligence, finding that the British plane was not transmitting an electronic signal from an onboard transponder used to identify whether an aircraft is friend or foe.

But British investigators reported shortcomings in the training of the operators of the Patriot anti-missile system as well as in the rules they were given to determine when to fire -- factors that the British probe cited as contributing to the downing.

The British Tornado GR4A aircraft was struck by a Patriot missile on the night of March 23 as it approached the Kuwaiti border, returning from a combat mission over Iraq. The plane's two-man crew died instantly, according to investigators.

Both the U.S. Central Command and Britain's Ministry of Defense released reports on the incident Friday after parallel inquiries. But in contrast to the U.S. report, whose major findings and recommendations were heavily redacted for publication, the British report publicly identified a number of weaknesses in the preparation and operation of the Patriot system.

The British inquiry found that codes in the Patriot system's computer lacked sufficient detail to interpret signals emitted from transponders on friendly aircraft.

"The criteria programmed into the Patriot computer were based on the many different anti-radiation missiles available worldwide and were therefore very broad," the British report said. "Criteria should have been much tauter, based on the known threat from Iraq."

The Patriot battery's "rules of engagement" also "were not robust enough" to avoid a friendly aircraft being wrongly classified as a missile, the report said. Additionally, much of the Patriot crew's training had dealt with recognizing generic threats, not those specific to Iraq.

Further, the Patriot unit was operating without all its communications equipment, which limited its contact with battalion headquarters and thus its view of aircraft operating in the region, the report said. The crew was driven to make a decision to shoot within one minute of detecting what it thought was a missile.

As for why the British plane was not emitting a proper identifying code, British investigators cited "power supply failure" as the most likely cause, and concluded that the plane's crew probably was unaware of the failure.

The Patriot anti-missile system was first pressed into service during the 1991 Persian Gulf War to defend against Iraqi Scud missiles. Although initial reports claimed considerable success, the system was later found to have missed most of its targets.

Since then, the Pentagon has invested several billion dollars to improve Patriot's performance with a stronger radar, advanced onboard computers and a new interceptor. Military officials credited the system last year with intercepting all nine Iraqi ballistic missiles that it went after.

But the downing of the Tornado was not the only major blunder. On April 2, a Patriot missile obliterated a U.S. F/A-18 Hornet, killing a Navy pilot. That incident remains under investigation.