WASHINGTON -- Human error and electronic failure to correct it led two U.S. fighter jets to shoot down two U.S. Army helicopters yesterday in broad daylight over northern Iraq.

The helicopters were carrying officials on a U.N. inspection tour of the Kurdish area in northern Iraq. All 26 crew members and passengers -- 15 Americans, three Turks, two Britons, one Frenchman and five Kurds -- were killed.

The incident, involving two U.S. F-15C pilots flying a United Nations peacekeeping mission over northern Iraq, created brief fears in Washington of new hostilities with Iraqi President Saddam Hussein when the downed helicopters were identified initially as Iraqi. The two fighter planes were enforcing the "no-fly zone" set up to protect the Kurdish minority from attacks by Mr. Hussein's forces.

The latest "friendly fire" tragedy sent the Pentagon into a state of shock and the nation into mourning, and raised questions about how such an error could occur despite state-of-the art technology intended to prevent it.

As the bodies of the victims were flown to Turkey, en route to their final resting places, President Clinton expressed "terrible sorrow" and pledged a thorough investigation. He ordered U.S. flags on public buildings nationwide to be flown at half-staff through sunset Monday "as a mark of respect for those who died as a result of the tragic incident."

When the shock of a major military bungle sank in at the Pentagon, Defense Secretary William J. Perry postponed a trip to Korea, due to start today, to take over the investigation into how the two helicopters were downed during a coordinated mission with the F-15s and under the control of an AWACS airborne control center circling above.

"I take full responsibility for today's tragedy," Mr. Perry said.

"And I pledge that I will take a direct role in ensuring that the investigation is conducted as thoroughly and as quickly as possible."

Mr. Clinton and other senior administration officials offered condolences to the families of the victims. The victims' identities were being withheld last night, pending notification of their families.

Three things about the incident are certain: The downed U.S. Black Hawk helicopters were wrongly identified visually as Iraqi Hind helicopters by the F-15 pilots; the electronic friend-or-foe identification systems on the helicopters failed to save them; and the AWACS overhead control aircraft did not avert the mistaken target identification.

Foolproof system

The system was supposed to be foolproof and had most recently been checked by the local commander of the U.N. operation to protect the Kurds in northern Iraq, Brig. Gen. Scott Pilkington, who flew similar missions over the area in a Black Hawk helicopter on April 11 and an F-16 fighter on April 13.

Stressing that the flights were routine, Lt. Gen. Richard Keller, chief of staff of the European Command, in Stuttgart, Germany, said yesterday, "It is not as if there were some heightened concern or some new threat."

The ill-fated mission had been rehearsed Wednesday, with all the air crews attending a briefing at which they were told which planes would be operating and where, and the radio frequencies to be used. The F-15 pilots had been flying similar missions since mid-February.

"This whole thing has been rehearsed ahead of time," General Keller said. "Procedures are in place . . . to preclude accidents like the one experienced today."

The U.S. planes were flying as part of the U.N.-sanctioned "Operation Provide Comfort" to protect the Kurds against Iraqi attack andharassment. The operation involves U.S., British, French and Turkish forces. It includes aid deliveries and the enforcement of a "no-fly" zone north of the 36th parallel from which Iraqi planes are banned.

Five questions will dominate the investigation:

* Were electronic IFF (identify friend or foe) systems switched on aboard the helicopters, identifying them as U.S planes?

* Did the F-15 pilots electronically try to identify the helicopters?

* If they did, did red cockpit lights warn the helicopter pilots that they were being targeted, enabling them to signal their identities?

* Were the helicopters and fighter pilots speaking with one another and with the AWACS controllers?

* Why did the F-15 pilots believe they had to fire quickly on two helicopters 35 miles inside the no-fly zone, even if they were Iraqi?

The last time the Iraqis violated the no-fly zone was on Jan. 17, 1993, when a U.S. F-16 shot down an Iraqi MiG-23 flying two miles north of the 36th parallel. Iraqi helicopters have never ventured far into the zone, the Pentagon confirmed last night.

Michael McCurry, the State Department spokesman, said there had been an "atmosphere of tension" for months in the Kurdish region,

The "black boxes," containing the flight data and conversations during the incident, have been recovered from all the planes and will be examined by experts at the U.S. Air Force base in Ramstein, Germany. The F-15 pilots and the commanders in the AWACS plane will also be questioned.

Military officials refused to give details of the rules of engagement for the mission or the line of decision to pull the trigger.

But some clues to the tragic confusion have already emerged. The helicopters were village-hopping, intermittently dropping out of the air-control loop to visit a village and then re-entering it en route to the next.

The helicopter pilot would be required to make radio and "squawk" (identification) contact with the AWACS each time he took to the air, General Keller said, briefing Pentagon reports by a radio link with his headquarters in Germany.

"Whether that contributed to the accident or not, I am not sure," he said. "But it's true he would have been in and out of the air as he accomplished his mission."

Loss sinks in

This could explain why it took the military some time to realize that the downed helicopters were U.S. planes. The first report of the shoot-down came at 9:30 a.m. in Iraq (3:30 a.m. EDT). According to General Keller, the mission controllers, who thought the downed helicopters were Iraqi, realized "probably within half an hour" that they had lost contact with the U.S. helicopters.

Another clue is that the Black Hawk helicopters assigned to the "Provide Comfort" mission are fitted with external supplementary fuel tanks for greater range. Those could be mistaken for the auxiliary wings attached to the Iraqi Hind helicopters. The configuration could be particularly confusing from above.

"The pilots of the F-15s feel they had positively identified the Hinds," General Keller said.

The U.S. Black Hawks were painted green, while the Iraqi planes are usually a desert tan, but the color distinction might have been masked by the early morning angle of the sun, according to the military officer.

The mission involving the two Sikorsky Black Hawk helicopters, the F-15Cs, and the AWACS command center began when the F-15s took off from a base in Turkey at 8:34 a.m. local time to patrol the no-fly zone. The helicopters left at 8.50 a.m. The weather was good, the visibility clear.

The fighters were "reported to be under the positive control" of the AWACS plane, which was monitoring the airspace over the no-fly zone, according to General Keller.

At 9.30 a.m., the F-15 pilots "visually identified what they thought were two Iraqi Hind helicopters, 35 miles north of the Iraqi town of Irbil. One of the F-15s fired an AIM-120 radar-guided air-to-air missile. The other fired an AIM-9 Sidewinder short-range, heat-seeking missile.