Pentagon officials who touted the Aegis ship defense system as the most sophisticated in the world are scrambling to explain why it couldn't tell the difference between a large Airbus jetliner and a far smaller F-14 attack jet.

That will be one of the issues examined by an official investigation team headed by Rear Adm. William N. Fogarty, who is en route to the Persian Gulf to find out what happened.The cruiser Vincennes shot down Iran Air Flight 655 in the mistaken belief that it was an attacking Iranian F-14, according to Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff.

The incident came shortly after the Vincennes had fired at Iranian patrol boats that U.S. officers believed had attacked the ship's helicopter, which was flying patrols in the Strait of Hormuz.

It was unclear whether the Airbus was descending in a threatening manner, as the Vincennes skipper apparently believed, or whether the jet actually was climbing when it was shot down.

The Washington Post quoted Pentagon sources Tuesday whom it did not identify as saying that a report filed Monday by another U.S. Navy ship, the frigate Sides, said the Iranian plane was climbing, as Iran has claimed. The Post said officials did not know whether the

report from the Sides, or the report from the Vincennes saying the Iranian plane was descending toward the ship in a pattern similar to one an attacking plane would make, was accurate. The Vincennes is equipped with the Aegis system, which the Navy has touted as the world's most sophisticated and complex electronic defense.

The European-built Airbus is far larger than the F-14, which the United States sold to Iran during the reign of the shah. The Airbus is 177 feet long, compared to only 62 feet for the F-14.

Crowe said that difference was less apparent because the Iranian plane was coming straight at the Vincennes. "One of the most difficult problems is from a radar blip, particularly from a head-on target, to identify the type of aircraft," he said.

"It's not that surprising a mistake," said a congressional source Monday, speaking on condition of anonymity. The darkened radar screens show only lighted blips, not images of planes in profile, said the source, who has witnessed demonstrations of the Aegis system.

The blips are smaller if the plane is coming head-on than they would be if the plane were scanned sideways, the source said.

A Pentagon source, who had also seen Aegis demonstrated, agreed. "You have to know how to read the screens. It's not spelled out in simple black and white, particularly if they didn't have the transponder on."

The transponder, also known as an "Identification Friend or Foe" device, or IFF, sends out continuous electronic signals detailing the identity of a plane. They are standard on all American commercial planes.

Crowe has said U.S. officers concluded that they were dealing with a military craft in part because of electronic transmissions from it. CBS News reported Monday evening that the Iranian plane was issuing two sets of signals - one civilian and one typically used by military craft.

Crowe said the Vincennes was looking specifically for F-14s. Iran only has four or five working F-14s, but they are the most capable warplanes in the Iranian inventory and thus are considered by the Navy to be the most threatening Iranian air weapon.

The working F-14s were recently moved from northern Iranian bases to Bandar Abbas, chiefly because Iran has suffered ground defeats in its war with Iraq. The Airbus took off from Bandar Abbas.