MILWAUKEE (AP) _ A jury found an Illinois contractor willfully violated federal safety rules, causing the deaths of three workers in a 1988 sewer tunnel explosion.

''This is a very important case for the construction industry, a very hazardous industry,'' Richard Fiore, an attorney with the Labor Department's regional office in Chicago said after Wednesday's verdict.

''I'm confident lives will be saved'' because the outcome will promote safer workplaces, he said.

Federal authorities alleged S.A. Healy Co. failed to shut off electricity in a Milwaukee sewer tunnel construction project after methane gas was detected there. They said a spark from a pump ignited the gas, killing three workers who had re-entered an evacuated area to make sure the work area was safe.

Defense attorney David Cannon argued during the 3 1/2 -week trial that Healy did not willfully violate any rules of the U.S. Occupational Safety and Health Administration. He said the company relied on the expertise of project engineers. Cannon would not comment after the verdict.

U.S. District Judge Terence Evans scheduled a hearing next Wednesday for consideration of post-verdict motions. He said sentencing would then be scheduled within 30 days. Healy could face $1.5 million in fines.

The federal conviction came a month after Healy was fined $15,000 in state court for two counts of negligent homicide in the same incident.

Karen Sochacki, sister-in-law of victim Rick Sochacki, said the end of the case came as a relief for family members, but it did not ''make up for the deaths.''

The federal complaint originally detailed a dozen OSHA violations that carried a maximum fine of $6 million. It was later amended to represent just three charges, one for each of the victims.

The jury found Healy caused each of the deaths Nov. 10. 1988 by willfully violating OSHA regulations that required:

-Proper training of employees concerning procedures of handling methane gas.

-Electrical power shut-offs when gas was detected in the tunnel.

-Proper installation of equipment for a hazardous work site.

The government also had alleged Healy violated OSHA safety regulations on ventilation in each of the deaths, but the jury did not find the company guilty of violating those specific regulations.

Healy's project manager for the tunnel, Patrick J. Doig, was originally charged by a federal grand jury with aiding and abetting Healy in the violations of OSHA regulations. But Evans dismissed those charges in January, ruling that federal law allowed only the employer to be held liable for the safety violations.