PETERBOROUGH, N.H. (AP) - A Massachusetts man who works at a New Hampshire ball bearings plant where an explosion injured at least 15 people says he heard a bang, felt a rumble and saw snow blown off the plant's roof.

New Hampshire Ball Bearings Inc. machine operator Paul Clark lives in Pepperell, Mass. He says he was outside the plant in nearby Peterborough, N.H., when he heard the explosion Monday afternoon. He says his girlfriend was inside working when shelves fell off the walls but she was uninjured.

He says he saw the plant's windows shattered and blown out.

Firefighters say the cause of the explosion hasn't been determined but it appears to be an industrial-related incident.

Company spokeswoman Kathy Gerrity says it's unclear when the plant will reopen.