Pentagon officials raised ``the possibility'' that a military identification signal received by the cruiser Vincennes may have been sent by a military jet and not the Iranian commercial airliner shot down by one of the ship's missiles, Rep. Les Aspin said today. But a Navy spokesman, Lt. Brian Cullin, said, ``We do not believe there was another aircraft in the area.'' Aspin, chairman of the House Armed Services Committee, said that possibility was raised Tuesday by Defense Department officials who met behind closed doors with House members to discuss Sunday's attack by the Vincennes, in which all 290 people on the Iranair jetliner died. Pentagon spokesman Dan Howard said Tuesday the Vincennes' sophisticated electronic equipment had picked up transmissions from Iran Air flight 655 on a military frequency, leading the crew of the cruiser to think it was firing at an Iranian F-14 fighter. The Pentagon officials who briefed the House members ``said they were examining a wide range of possibilities about what happened,'' Aspin said today. ``We pressed them on the military transmission angle, the fact that the Vincennes said it was receiving on a military channel'' from the airliner, he said. ``We were told that there were three possibilities. One is that the Airbus had in fact been broadcasting on both civilian and military frequencies; the second is the fact that people on the Vincennes made a mistake in reading the information, and the third is that there was a second plane that was a military plane,'' he said. ``The possibility exists that there was a military plane that was somehow masked'' by the airliner, he said. The Identification Friend or Foe (IFF) system used by the Vincennes as part of the identification process ``is good, but it also has a lot of limitations,'' said Aspin, a Wisconsin Democrat. ``For one thing, it's not very good at determining altitudes,'' Aspin said. He said that if there were a second plane, its existence could be one explanation of the wide divergence between reports about the altitude of the Airbus. The Vincennes thought the Airbus was at about 7,000 feet, but Howard said Tuesday that the USS Sides, another ship in the area, had tracked the jetliner's altitude at above 12,000 feet. The Vincennes efforts to identify the plane were hampered by the lack of an airborne radar system in the vicinity, said Adm. William Crowe, the chairman of the Joint Chiefs of Staff who on Sunday acknowledged that downed plane was an airliner. ``There was no air cover,'' Crowe said Sunday. The United States does operate E-2 Hawkeye and Airborne Warning and Control (AWACS) reconnaissance aircraft with sophisticated look-down radar in the region, but they were not aloft at the time of the incident. Crowe was asked on Sunday whether any other Iranian aircraft were in the area, and said ``there was an F-4 up later. Whether it had been up all the time or earlier, we're not able to determine. ... It didn't play a part in this.'' The F-4's and F-14's were among the U.S.-made weapons sold to Iran during the reign of Shah Mohammed Reza Pahlavi, who was overthrown in 1979.