“HOW DID YOU let this happen?” That agonizing question from relatives of the 29 men killed in the 2010 explosion of West Virginia’s Upper Big Branch coal mine spurred a state investigation that has now delivered its stark answer: A mine owner was reckless with the safety of its workers, and state and federal regulators failed to do their jobs. The unsparing conclusion should be a prod for industry and government.

The report of the independent team appointed a year ago by then-Gov. Joe Manchin III (D) (now a U.S. senator) says this accident could have been prevented. It lays fault for the nation’s worst mining disaster in 40 years squarely with the owner of the mine, Massey Energy. Investigators led by former federal mine safety head J. Davitt McAteer detailed a pattern of negligence — from inadequate ventilation to faulty equipment to indifferent control of dangerous coal dust — and said that Massey management fostered a culture where “wrongdoing became acceptable.” Workers learned not to complain about safety issues, lest their life be made more difficult, the team found.

Underscoring this are the sadly prophetic words of miner Gary Wayne Quarles that open the report. “When I get up in the mornings, I don’t want to put on my shoes. I don’t want to make myself go to work. I’m just scared to death something bad is going to happen.” This is what the 33-year-old father of two told a friend a day before he died in the April 5 explosion.

Massey officials issued a statement disputing many of the report’s findings. The firm contends that an unpreventable burst of methane gas caused the explosion, but investigators produced evidence they say rules out that theory. The blast is subject to ongoing investigations, which, based on these findings, should result in criminal and civil penalties against the company and its managers, many of whom declined to cooperate with West Virginia’s inquiry.

The investigators also found that state and federal watchdogs failed miserably. Neither the federal Mine Safety and Health Administration nor the West Virginia Office of Miners’ Health Safety and Training used all the tools at its disposal to ensure the company complied with federal laws. In some cases inspectors took appropriate action, but their superiors failed to follow through.

America’s reliance on coal is not likely to end anytime soon. Mr. McAteer is right to say that “we are long past the time where we can accept the loss of 29 miners.” A good start would be to pass the mine safety legislation that is stalled in Congress.