WASHINGTON – The Justice Department is conducting a criminal investigation of the explosion that killed 29 people at a West Virginia coal mine, and current and former employees of the mine’s owner have been interviewed by the FBI, law enforcement sources said Friday.

The probe, which is at least a week old, is focused on the deadly April 5 blast and the circumstances surrounding it, the sources said. It is still in its early stages, and officials do not yet know if it will lead to charges, the sources said.

The Upper Big Branch mine in Montcoal, W.Va., is owned by Massey Energy Co. It had been cited for numerous safety violations, including a dozen in the weeks ahead of the explosion for problems with ventilating the mine and preventing a buildup of deadly methane.

NPR News first reported on the investigation Friday, saying that the FBI is looking into potential criminal negligence on Massey’s part and into possible bribery of officials of the Mine Safety and Health Administration, the federal agency that regulates the mining industry.

But law enforcement officials told The Washington Post that it is far too early to identify anyone as a target of the probe, and that investigators need to sift through more evidence before making that determination. And an Obama administration source said MSHA is not a target.

Massey officials have said they did not know what triggered the explosion. In a statement, the company said it had no knowledge of any criminal wrongdoing and is cooperating with multiple investigations of the blast.

“It is not uncommon that an accident of the size and scope of UBB would lead to a comprehensive investigation by relevant law enforcement agencies,” Massey said in a statement. “Massey does not and will not tolerate any improper or illegal conduct and will respond aggressively as circumstances warrant.”

The revelation of a criminal investigation escalates the controversy over the explosion, which has prompted much examination of Massey’s safety record and denials of culpability by company officials.

President Obama vowed at a memorial service this week to get to the bottom of the tragedy, and Joseph Main, who heads MSHA, told Senate lawmakers that Massey appeared to take a “catch me if you can” approach to safety at the mine.

He said the agency’s efforts were frustrated by Massey’s tendency to appeal citations, which prevented the imposition of tougher rules applicable to companies deemed to have a “pattern of violations.” Company officials have denied any negligence.

Mine safety experts said the Justice Department’s decision to act is uncommon.