An explosion at a New Hampshire ball bearings plant Monday injured at least 13 people, two critically.

Peterborough Fire Department spokesman Eric Bowman said that the two critically injured people were airlifted to Massachusetts hospitals for further treatment. Four others were described as being in serious condition. Bowman did not specify what types of injuries were sustained.

Monadnock Community Hospital spokeswoman Laura Gingras told the Associated Press that 13 people were treated there. She says six were released by Monday evening.

Company spokeswoman Kathy Gerrity says it's unclear what caused the explosion Monday afternoon at New Hampshire Ball Bearings Inc. in Peterborough. Bowman said late Monday that there were no signs of criminal activity and authorities were describing the explosion as an "industrial-related incident." Bowman acknowledged that the plant used potentially explosive chemicals for processing, but did not specify any types of chemicals. 

The southwest New Hampshire plant employs 700 people. Gerrity says she's unsure how many people were inside when the explosion happened Monday afternoon but there are usually about 450 working around that time.

The explosion, which was reportedly felt over a mile away, blew out most of the windows on the first floor of the plant. Firefighters were assessing the building's structural integrity and the investigation was expected to carry on through Tuesday. 

New Hampshire Ball Bearings' corporate headquarters are in Chatsworth, Calif. It's a division of Japanese company Minebea.

The Associated Press contributed to this report.