WASHINGTON -- The failure of two U.S. Black Hawk helicopters to respond to an electronic identity check by the U.S. F-15 pilots who shot them down Thursday over northern Iraq yesterday became the focus of the investigation into the "friendly fire" tragedy.

The helicopters' silence after the "friend or foe" inquiry from the U.S. fighters raises two possibilities: mechanical failure or operator error in either the fighters or the helicopters.

What appears clear, according to an initial report to Gen. John Shalikashvili, chairman of the Joint Chiefs of Staff, is that the lack of a "friendly" response from the helicopters reinforced the fighter pilots' mistaken visual identification of them as Iraqi Hind helicopters.

That led to the decision by the pilots to fire two missiles at the helicopters, killing all 26 crew members and passengers -- 15 of them Americans -- who were on a United Nations mission to protect Kurds from Iraqi persecution.

The rules of engagement for the F-15 flight, according to the general, did not require the pilots to give a warning of attack to the helicopters once they were identified as hostile.

The most baffling aspect of the mystery is that whatever went wrong was duplicated. Either equipment broke down in both helicopters or both fighters, or operator failures occurred simultaneously in two of the four cockpits. In addition, the planes could have contacted each other by radio.

"How could this have possibly happened?" a perplexed Defense Secretary William J. Perry said yesterday at a Pentagon news conference that left as many questions as it provided answers.

Mr. Perry said an Air Force-led investigation into the incident would uncover any personal culpability, systems failures or breaches of operating procedure.

"If individuals are found to be culpable, we will discipline them," he said. "If our procedures need changing, we will change them, and we will change them immediately."

One change in the allied mission to protect the Kurds in northern Iraq had already been ordered, he said, declining to give details. The United States also suspended air patrols over northern Iraq for one day to reinforce the safety procedures that failed Thursday.

The revelation that the F-15 pilots had used their IFF (Identify -- Friend or Foe) identification system on the helicopters before firing appeared to exonerate the pilots from one possible lapse in procedure.

But it left two crucial questions unanswered: How did they mistake the U.S. Black Hawks for Soviet-made Hinds in two visual fly-by checks? And what convinced the F-15 pilots that the helicopters were such a danger that they should be shot down immediately?

Implicit in the failure of the helicopters to give a "friendly" response to the fighters was the possibility that the transmitting equipment in the helicopters or the receiving equipment in the fighters was not working properly, or was not programmed properly. In the helicopters, the transmitters might simply not have been switched on.

Interviews with an F-15 pilot who flew on coordinated missions with helicopters during the Persian Gulf war, a Black Hawk pilot and another Army helicopter pilot produced this assessment of the possibilities:

* Mechanical failure: This presumes that the operators did everything correctly but that the communications link between the F-15s and the helicopters broke down. The flaw in this theory is that it would mean that equipment in both cockpits at either end of the link would have failed at the same time. The chances of that are regarded as remote.

* Operator error: This presumes that the equipment was working but was not properly used. It could have been miscoded. The codes are usually changed daily or by mission, and are entered into the "friend of foe" transmitting system by the pilot or the ground crew chief. The pilot is responsible for checking the code's accuracy.

The chances of two separate systems being miscoded are remote, according to the experts. Even if they were, this would be unlikely to provoke an attack, the F-15 pilot said, because the response would still be recognizable, even if incorrect.

Another possible error is that the manually activated IFF systems in the helicopters were turned off.

When a flight of aircraft -- either fixed-wing fighters or rotary-bladed helicopters -- takes off, the flight leader frequently orders only one of them to use its IFF system to minimize electronic interference.

That single plane would then check with ground radar control and the AWACS to make sure its identification signal was strong and clear. If it wasn't, another plane would be designated to switch its system on.

It is not known if this happened with the two helicopters Thursday.

Both helicopters pilots also might have turned off their IFF transmitters, because the transponders provide a radar "lock-on" the inquiring plane that can be used to target a radar-guided missile.

"During a hostile situation, turning your IFF off is the tactically smart thing to do once you are in enemy territory," said the F-15 pilot, who was interviewed at the Pentagon on the condition of anonymity.

The flight recorders, which will show whether equipment was operating when the helicopters were hit, are being examined at the U.S. air base at Ramsteim, Germany.

The helicopters were shot down 25 miles inside the "no-fly" zone enforced by U.S., British, French and Turkish forces to prevent Iraqi planes for attacking Kurdish villages in northern Iraq. The last known incursion into the zone by an Iraqi plane was in January 1993.

Overarching the communication -- or lack of it -- between the F-15s and helicopters was the electronic surveillance and control of the entire "no-fly" zone by an AWACS airborne control center.

The central question involving its role is why it did not identify the helicopters as U.S. Black Hawks and prevent the attack -- whatever the F-15 pilots thought they saw or concluded from the helicopters' silence.

"We don't have definitive answers to these question at this time, but we will have in a very short time," Mr. Perry said yesterday.

Noting that the F-15s, the Black Hawks and the AWACs had three ways of communicating -- visual, electronic and radio -- General Shalikashvili said: "They didn't have to all operate. Just one had to operate, and this accident would not have happened."

He noted that 2,700 fixed-wing flights and 1,400 helicopter flights had been successfully completed under the same procedures as Thursday's flights since the "no-fly" zone was established in October 1991.

The two U.S. F-15s were part of a 34-strong flight of allied planes rotated in and out of the zone Thursday under the overall command of the AWACS. The Black Hawks were carrying a humanitarian mission to meet with Kurdish officials.