Early Warning Systems Probed After Downing : Military: Signals were picked up minutes before F-16 was hit over Bosnia. Pentagon wants to know why U.S. pilot Scott F. O’Grady was not alerted.

The Pentagon is looking into whether NATO early warning radar planes flying close to Air Force Capt. Scott F. O’Grady’s F-16 jet fighter on June 2 had enough time to warn him that reconnaissance aircraft had discovered Bosnian Serb missile batteries in the area.

Pentagon officials say that is a critical question remaining in the mystery over why O’Grady apparently had no warning before a rebel Serb SA-6 mobile missile battery shot his plane down over northwestern Bosnia-Herzegovina, leading to a nearly six-day search that resulted in his rescue June 8.

U.S. officials said Thursday that an American U-2 spy plane banking over the area had detected the SA-6 batteries electronically barely four minutes before O’Grady’s F-16 was hit.

A second F-16 nearby also picked up a similar signal even closer to the time O’Grady was shot down.

The information reportedly was relayed instantly to U.S. intelligence officials at the National Security Agency and transmitted immediately to a NATO Airborne Warning and Control System, or AWACS, plane that was serving as air controller in the region.

But officials say the data gathered by the U-2 suggested that the evidence of a possible SA-6 battery was only fragmentary, and it is unclear whether the AWACS crew would have had time to process it and send a warning to O’Grady before his F-16 was hit.

Officials familiar with the after-action reports now coming in from the field say the SA-6, a Soviet-made surface-to-air missile, emits tracking signals only intermittently--rather than continuously--to avoid detection.

As a result, they say, such fragmentary information can seem unreliable in the heat of action and sometimes does not firmly point to a missile threat until minutes later, after experts have had time to process and analyze it carefully.

Pentagon officials are also looking into whether the internal sensing systems aboard O’Grady’s F-16 that were designed to detect missiles after they are launched were able to pick up the SA-6 missile.

In Naples, Italy, on Thursday, experts from the North Atlantic Treaty Organization, analyzing the incident, said they believe that the shoot-down was an ambush plotted by the Bosnian Serbs in retaliation for NATO’s bombing of an ammunition dump near the rebel Serb capital of Pale a few days before.

They said the attack was part of a three-pronged response that also included shelling U.N.-declared Muslim “safe areas” and seizing U.N. peacekeeping troops as hostages. The last of those hostages were to have been released this week.

U.N. officials had asked NATO to destroy the depots after Serb nationalists flouted a U.N. order to return weapons they had removed from U.N. collection points.

“The Serbs knew that NATO wouldn’t . . . expose themselves . . . so they decided to go to NATO,” one officer said.

The Defense Department declined to comment on its review of the AWACS crew’s performance.

Pentagon spokesman Kenneth H. Bacon told reporters that the department will compile a report after it completes its investigation and will send it to Congress in mid-July.

The concerns outlined by officials appeared consistent with assertions by Defense Secretary William J. Perry earlier this week that the success in shooting down O’Grady was not caused by a failure in intelligence-gathering, as some had alleged.

Perry told a congressional subcommittee Tuesday that, “based on the data I’ve seen so far, I think we did a good job on collecting the signal.”

But he added: “I’m not satisfied at this point that we did the best job in transmitting and relaying the information to the person who most needed the information”--a reference to O’Grady.

The suggestion that the shooting down of O’Grady was part of a trap set by Bosnian Serbs was voiced last week by U.S. and NATO officials, but the reassertion by allied officials on Thursday appeared based on additional evidence.

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NATO officials in Naples also gave the following description of the Bosnian Serb ambush:

* A tracked vehicle carrying three SA-6 surface-to-air missiles was dispatched to lie in wait in a rural area north of the Serbian city of Banja Luka, close to a town that NATO maps show as Bos Petrovac, not far from where O’Grady was rescued. The Bosnian Serbs already have strong missile defenses at Banja Luka to the south, and the Croats maintain batteries farther north. As a result, NATO planes began flying in what had been a relatively safe corridor between them--a route that was well-watched by the rebel Serbs.

* O’Grady and his wingman were flying at 27,000 feet on routine patrol June 2 when they ran into trouble. NATO officials say they caught a momentary flicker from what might have been a missile battery radar, but the trace vanished quickly. A few minutes later, the missile battery locked onto targets and fired two radar-guided weapons, NATO officers say.

O’Grady’s wingman was reporting the incident just as the missiles broke through the clouds below.

* Although the first missile passed harmlessly between the two fighters, the second hit O’Grady’s plane a few seconds after his wingman caught sight of it. O’Grady ejected safely, but his wingman did not see him get away from the burning fighter aircraft.

NATO would have no firm word from O’Grady until the pre-dawn hours of June 8, when he radioed for help to another patrolling F-16.

He was rescued later that morning by Marines accompanied by about a dozen NATO aircraft.

U.S. officials announced last week that, in the wake of the shooting down of O’Grady’s plane, allied commanders have ordered that all NATO fighter aircraft flying over Bosnia be accompanied by escort planes equipped to detect and destroy missile batteries.

Pentagon officials said they also are considering ways to speed the amount of time it takes for AWACS aircraft to process intelligence data and transmit it to field commanders and to aircraft flying over Bosnia.

O’Grady is currently resting in the Washington area.

Pine reported from Washington and Montalbano from Rome.