BOSTON (CBS) – The company at the center of the Merrimack Valley gas explosions and fires is now facing a criminal investigation.

NiSource, the parent company of Columbia Gas, revealed Thursday it was served a grand jury subpoena by the U.S. Attorney’s Office in Boston back on September 24, just 11 days after the disaster.

Columbia Gas accidentally over-pressurized a gas line on September 13, causing explosions and fires in Lawrence, Andover and North Andover. A young man was killed when a home blew up and a chimney crushed his car.

Thousands of customers still have no heat or hot water. The company says it plans to have gas restoration complete by December 16.

NiSource and Columbia Gas say they are cooperating fully with the investigation, which became public Thursday when NiSource released it’s third quarter financial report. Within that report was a filing with the Securities and Exchange Commission and the criminal investigation was mentioned in that file.

The U.S. Attorney’s Office would not confirm or deny the investigation.