Federal Aviation Administration officials said Tuesday that three Soviet Bloc trainers had been flying in formation during the celebration when one suddenly veered off, flew over a park and slammed into Old Town Road, missing nearby houses. The impact killed David Zweigle, 42, the city's airport manager, and Robert Chamberlain, 63, of Morrison, Colo., a retired airline pilot and former U.S. Air Force test pilot, who set numerous records in the B-1B bomber. Zweigle owned the restored Aero Vodochody L-29 Delfin, a 1960s-era jet built in Czechoslovakia. Ian Gregor, an FAA spokesman in Los Angeles, said the pilots had not applied for clearances to fly over populated areas for the Techachai celebration, which was attended by hundreds of people. Gregor said special permission might have been required for the L-29s, because they are designated by the FAA as 'experimental, exhibition' planes, which are generally prohibited from flying over densely populated areas except for takeoffs and landings. The designation is a safety measure and assigned to aircraft that are neither built to U.S. standards nor assembled at FAA-certified factories. As part of the agency's investigation, Gregor said FAA officials were interviewing witnesses and reviewing a videotape to determine whether the pilots were flying below the minimum required altitude of 1,000 feet. 'Speaking generally, both these things are serious,' Gregor said. 'Violations of this nature would merit a penalty on the higher end of the scale.' Those penalties include suspension and revocation of pilots' licenses. Douglas E. Gilliss, a veteran pilot who flew one of the L-29s at Tehachapi, defended the flights, saying the planes avoided populated areas and flew between 1,200 and 1,500 feet, well above the minimum required altitude. Gilliss flies with the 'Thunder Delfins,' a group of L-29 enthusiasts. Gilliss said the L-29s flew along the area's railroad tracks and not directly over the city of about 35,000. 'This is a standard display up there. We've done them without a gripe from anyone,' said Gilliss, who lives in San Diego County, Calif. 'We didn't present any hazard to the public, and the flights were in the spirit of FAA regulations that encourage the exhibition of experimental aircraft.' 