WASHINGTON (AP) — Federal prosecutors said Friday they are investigating whether there was "willful criminal activity" by the company that operates the West Virginia coal mine where 29 workers died in an accident last month.

The U.S. attorney's office for the Southern District of West Virginia said in a letter that investigators are looking into possible criminal conduct by the mine's operator, Performance Coal, and its directors, officers and agents.

The letter, obtained by The Associated Press, asks the Labor Department to hold off pursuing dozens of civil cases against Performance for alleged mine safety violations.

Performance is a subsidiary of Massey Energy Co., which owns the Upper Big Branch mine.

Last month, federal law enforcement officials said the FBI had interviewed nearly two dozen current and former employees of Massey in the probe. But the Justice Department declined to publicly confirm there was a criminal investigation.

In the letter Friday, Assistant U.S. Attorney R. Booth Goodwin II said his office wants to make sure the pending civil cases don't interfere with the criminal probe. He asks Douglas White, a top lawyer for the federal Mine Safety and Health Administration, to delay 44 civil penalty cases against Performance that could form the basis of criminal penalties under federal mine laws.

The cases involve about 500 citations issued between June 3, 2006, and April 5, 2010 — the day of the explosion, according to a federal official familiar with the records. The official requested anonymity because he was not authorized to discuss the case.

About 300 of the citations are for "significant and substantial" violations, which are among the most serious that can be alleged at a mine, the official said. More than 90 allege the mine operator's high negligence or reckless disregard for safety standards.

The citations range from inadequate roof support and ventilation problems to improper accumulations of combustible dust, the official said.

A Massey spokesman has said previously that the company was not aware of the nature of the federal probe but that "we intend to cooperate in all phases of the accident investigation."

Massey chief executive Don Blankenship is set to testify next week before a Senate panel looking into the accident.

The company was repeatedly cited for problems with its methane ventilation system and other issues in the months before the accident.

A half dozen of the civil cases federal investigators are looking at have been pending for at least three years at the Federal Mine Safety and Health Review Commission, a separate agency that resolves mine safety cases.

Some lawmakers blame Massey for clogging the system with legal challenges to dozens of safety citations to delay stronger penalties. The commission has a backlog of more than 16,000 cases and lawmakers are trying to direct more money so it can hire more staff.

Massey has defended its safety record and says all of its appeals have merit.

MSHA Director Joe Main said at a Senate hearing last month that Massey has a "troubling record" of safety violations and appeared to take a "catch-me-if-you-can" attitude toward workplace safety at the mine.