A Washington-based public-interest law firm filed a class-action lawsuit yesterday against the five firms that make all-terrain vehicles (ATVs), seeking to force refunds for the 2.3 million individuals who purchased the off-the-road vehicles on the assumption that they were safe.

The suit, filed in U.S. District Court in Philadelphia, comes two weeks after the Justice Department announced a consent agreement with the five firms under which they will cease sales of three-wheeled ATVs.

That action evoked strong criticism from consumer groups, which accused the government of retreating from a threat to force refunds for the ATVs, described by some safety experts as "killer machines."

"Once again, plaintiffs' trial lawyers have stepped in to protect consumers when the government has failed to do its job," said Arthur H. Bryant, executive director of the Washington group, Trial Lawyers for Public Justice. "These machines are ticking time bombs. Every three days, two people die from using them."

Named as defendants in the lawsuit are the four Japanese firms -- Honda, Yamaha, Suzuki and Kawasaki -- that produce most of the ATVs sold in the United States, and Polaris Industries, L.P., of Minneapolis.

The suit accuses the five firms of violating federal racketeering statutes and state consumer laws, fraud and breach of warranty in selling the vehicles, designed for off-road conditions.

"They promised it was safe, and they have not been telling the public anything about the perils," said Robert M. Hausman, an attorney with the Washington group. "Their ads on TV just show happy trails."

The suit did not mention a specific amount of damages sought, but Hausman noted that under federal racketeering statutes, the firms could be liable for damages of as much as three times the purchase price of the ATVs.

James V. Lacy, general counsel of the Consumer Product Safety Commission, said the suit "should lay to rest any arguments by critics that the federal government's action against ATV manufacturers limits" owners' rights to sue the industry.

Industry officials have maintained that the vehicles are safe and that accidents have resulted from operator error.