The FBI backed away from six years of unqualified denials to Congress and the public Wednesday and conceded that it used "pyrotechnic" tear-gas canisters on the final day of the 1993 standoff with the Branch Davidian cult near Waco, Texas. The bureau said the devices were "pyrotechnic" only in a limited sense and bounced harmlessly off a concrete structure six hours before the compound's main building, built of wood, erupted in flames. About 80 people, including 24 children, were found dead after the fire. The revelation, after repeated denials, that federal agents used pyrotechnic devices of any definition was deemed so serious that Attorney General Janet Reno and FBI Director Louis Freeh on Wednesday night ordered an exhaustive inquiry into the events of April 19, 1993. About 40 FBI agents have been assigned to the review, and everyone who was at Waco that day will be re-interviewed, said John Collingwood, an FBI spokesman. "We continue to believe that law enforcement did not start the fire," Collingwood said. But there were clear signs Wednesday evening that the backtracking had spawned a credibility crisis for the FBI and the Justice Department. "This new information requires a thorough investigation of whether the Justice Department has misled the American people, and the Congress, about what happened at Waco," Rep. Dan Burton, R-Ind., who heads the Committee on Government Reform, said Wednesday. "I intend for the Government Reform Committee to get to the bottom of this matter." Another FBI spokesman said earlier Wednesday that the devices in question were "pyrotechnic" only in a limited sense _ that is, that they generated heat even as they dispensed tear gas. He said they were fired at the concrete "bunker" about 100 yards from the main wooden structure after less potent tear gas canisters failed to pierce the concrete. The federal agents wanted to get tear gas into the bunker because they feared the Davidians would try to escape through a tunnel between it and the wooden building, the spokesman said. The two pyrotechnic devices, military-type gas canisters known as M-651 grenades and fired by an FBI agent from a range of about 40 yards, also bounded off the bunker roof and into a nearby puddle, where they lay harmlessly, he said. After flames consumed the wooden building, the remains of the cult leader, David Koresh, and about 80 of his followers were found. Autopsies showed that some had died of self-inflicted gunshot wounds, others from the flames or smoke. A few escaped. The FBI spokesmen, while insisting that there is no new information to challenge the finding that the Davidians started the fire, acknowledged Wednesday night that the necessity to "recant and modify" earlier statements was acutely embarrassing. Reno came under heavy criticism immediately after the raid, but then regained respect as she accepted responsibility and answered questions with seeming candor. Her credibility and that of the FBI are at stake with the disclosures about the use of the military-type gas grenades. Freeh was not the FBI director at the time. The kind of force used by federal agents on April 19, 1993, could be critical in a wrongful-death suit filed by some Branch Davidian survivors and relatives of the dead and scheduled to open Oct. 18. But the FBI and Justice Department are far more worried that the recantation will fuel the suspicions of people who have long doubted the federal government's motives and actions in the incident. "We're fighting the conspiracy theorists," one bureau spokesman said. The FBI explained Wednesday night that on April 19, 1993, a bureau agent was driven to within 40 yards of the Davidians' concrete bunker in an Army Bradley fighting vehicle, then fired the two canisters. He was 100 yards from the wooden structure and was firing away from it, the bureau said. The unsuccessful use of those two canisters was noted in documents at the time, the bureau said Wednesday night.