IDAHO FALLS, Idaho -- Buried in underground tanks and dumped into trenches at the Idaho National Engineering and Environmental Laboratory northwest of here is a witches' brew of deadly chemicals and radioactive waste left over from the cold war. It includes enough plutonium debris from the Rocky Flats weapons plant in Colorado to build hundreds of nuclear bombs.

 This laboratory, more than half the size of Rhode Island, has built and tested civilian and military nuclear reactors for 52 years. Because of its residues and stored wastes, it was designated a Superfund site in 1989, and the government started trying to clean it up. Now two men who audited that effort say in a federal lawsuit that government contractors who were paid hundreds of millions of dollars made the contamination worse. When the auditors complained, they said, they were harassed until they resigned.

 The auditors said the contractors deliberately bypassed safety measures, turned off monitors and alarms, falsified documents, did not report spills, dumped hazardous wastes on the ground and illegally sent waste from a pit contaminated with plutonium to a public landfill.

 Those contentions shed a different light on what state and federal officials told the public about the contractors' problems at the site. And in internal documents, federal officials shared some of the auditors' concerns.

 Officials at the Idaho Division of Environmental Quality announced in May 1999 that the United States Energy Department, as the site owner, would pay $504,000 in fines and costs for mishandling dangerous waste. The division, and Energy Department officials, said at the time that the violations resulted from oversights or from problems created before the contractors took over.

 "Things happen," Mike Gregory, the hazardous waste enforcement coordinator for the state, said in an interview. "Someone gets lazy. Or they think they're doing right."

 But in 1998, an internal Energy Department review said the contractor that oversaw the lab and ran the cleanup at that time, Lockheed Martin Idaho Technologies, had not established "an underlying culture of rigor, discipline and sustaining leadership" on environmental, safety and health issues.

 That review, signed by John M. Wilcynski, manager of the Energy Department's Idaho Operations Office, said that three major accidents, including the deaths of two workers, had occurred. He recommended that the contract be put out for bid.

 Jim Fetig, a spokesman for the Lockheed Martin Corporation, based in Bethesda, Md., said that there might have been environmental missteps in Idaho, but that none were intentional.

 "I don't think for a second that there was an ethos of nonconcern about environmental issues," Mr. Fetig said.

 Waste was stored improperly in some cases, he said, but it was hard to find out what old storage tanks contained. "They are still trying to get a handle on what's in those tanks and what to do with it," Mr. Fetig said. "There was a lot of bad stuff out there and only so much money. A contractor can only do what the Department of Energy approves."

 Lockheed Martin did not seek to renew its contract in Idaho, but still manages two research facilities for the Energy Department, the Sandia National Laboratories in Albuquerque and the Knolls Atomic Power Laboratory in Schenectady, N.Y.

 Besides Lockheed Martin, which ran the site from October 1994 through September 1999, the lawsuit names EG&G Idaho Inc., the contractor that ran the site from 1976 to 1994, and the Westinghouse Idaho Nuclear Company, which ran the Idaho Chemical Processing plant from 1989 to 1994. The plant stored and reprocessed nuclear waste from reactors around the world.

 Mark J. Meagher, a Denver lawyer representing Westinghouse, said the company denied all the charges. Edward W. Pike, an Idaho Falls lawyer representing EG&G, declined to comment on the case.

 Also named is Coleman Research, a Lockheed Martin subcontractor that employed the auditors, Neil Mock and Scott Lebow. William Goodrich, a lawyer for Coleman, said the company did not retaliate against the men for their complaints.

 The lawsuit is being brought under a federal law that allows individuals who contend that contractors committed fraud to sue on the government's behalf and recover 25 percent to 30 percent of any judgment. The suit was filed in 1996 in United States District Court in Pocatello, Idaho, but was kept under seal until the government decided three years ago not to join it.

 The Energy Department considers the Idaho laboratory, now managed by BWXT Idaho, a consortium led by Bechtel Inc., essential to the future of nuclear power, both civilian and military. The laboratory has also been named to lead development of new cleanup technologies.

 The auditors arrived at the laboratory in the early 1990's. Mr. Lebow was a senior environmental, safety and quality regulatory compliance specialist. Mr. Mock was a senior scientist.

 They said that they were told that employees of Westinghouse and Lockheed Martin had turned off spill alarms on 300,000-gallon tanks containing liquid high-level radioactive waste, and that no one responded to two spills they saw in 1995.

 They said Lockheed Martin tried to flush out four other storage tanks EG&G had described as empty. When the tanks were found to contain corrosives contaminated by mercury at a rate nine times the reportable level, Lockheed Martin continued flushing the contaminated water -- 2.4 million pounds in all -- into a pond for absorption into the soil and for evaporation.

 Brad Bugger, a spokesman for the Department of Energy in Idaho, said that the mercury spill was an example of bad management but that it posed no additional risk to the environment because "only a couple of ounces of mercury" were involved.

 Mr. Mock and Mr. Lebow charge that from 1995 to 1998 Lockheed Martin employees occasionally disabled or disconnected the monitoring devices on smokestacks at a plant where high-level radioactive waste was processed, to conceal excess emissions of iodine-131, a radioactive isotope that is readily absorbed by the  human body. They say this happened at the laboratory's Nuclear Technology and Environmental Complex, formerly known as the Idaho Chemical Processing Plant.  In 1995, they said, monitors were also disabled at the laboratory's Waste Experimental Reduction Facility, which burned radioactive paper, clothing, plastic and garbage. That incinerator was closed last October after citizens threatened to sue Idaho officials, who denied it a hazardous waste permit.

 When Lockheed Martin managers were told in writing about the disabling of the air pollution monitors, the auditors say, the company told staff members not to report these acts to the authorities. Lockheed Martin denies this.

 Boxes of soil contaminated with hazardous waste, improperly labeled as low-level waste, were sent illegally to a disposal site in Utah in 1996, Mr. Mock and Mr. Lebow say, and a Lockheed Martin subcontractor, Lockheed Martin Advanced Environmental Systems, illegally disposed of waste from Pit 9, which contains plutonium and other radioactive substances, in the Bonneville County landfill.

 This month, Gary Johnson, the assistant inspector general of the Environmental Protection Agency, said he would look into questions about the laboratory raised by local environmental organizations concerned about airborne emissions. Another local organization has warned of dangers to the Snake River Aquifer, which is the water supply for 20 percent of Idaho's population.