Three workers killed in an explosion while checking gas levels in a sewer tunnel were acting as guinea pigs, says a safety advocate who calls the project's fatality rate the nation's worst per dollar spent.

"I'm absolutely convinced that this particular tragedy is inexcusable and absolutely avoidable," Joseph Kinney, executive director of the National Safe Workplace Institute in Chicago, said of Thursday's accident. "It did not need to happen."A city official defended safety procedures and said investigators did not know what triggered the blast.

In all, six workers have died since 1986 in the $2.1 billion deep-tunnel sewer project, the largest public works project in Wisconsin history. Last March, the federal Occupational Safety and Health Administration cited the contractor for inadequate ventilation.

The explosion occurred around 9 a.m. after the three went into the tunnel to check a high level of methane, a gas formed by natural decomposition, officials said. The methane had caused workers to evacuate.

Rescue workers, who had to clear explosive fumes and remove twisted wreckage while contending with poor lighting and knee-deep mud, took eight hours to remove the bodies.

"It was a mess," said fire Capt. Bert Bemann. "It was like hell."

The victims were identified as Ronald Kohne, 49, a tunnel superintendent from Big Sandy, Tenn.; Rick Sochacki, 34, a safety officer from South Holland, Ill.; and Tony Bell, 25, of Sturgeon Bay, Wis., a shift foreman.

Autopsies were scheduled to determine what killed the men. District Attorney E. Michael McCann began an investigation.

"It seems to me that the three men were the modern-day equivalent to canaries in cages in the old coal mines," said Kinney, referring to the old practice of using canaries to check for lethal gas levels in coal mines. If a canary died, the miner knew to get out.