LAWRENCE, Mass. -- A federal investigator says there's no evidence to suggest the gas explosions that rocked communities north of Boston were intentional.

National Transportation Safety Board Chairman Robert Sumwalt said Saturday as federal officials opened their investigation into Thursday's disaster that there doesn't appeal to be "anything nefarious, anything suspicious, anything intentional."

A series of explosions and fires killed a teenager, injured dozens, damaged homes and forced thousands of people to evacuate.

Sumwalt said it appears Columbia Gas' pipeline control centre in Columbus, Ohio registered a pressure increase in the Lawrence area.

He said investigators will be working to develop a timeline, including how local utility officials reacted to the pressure increase.

Investigators expect to be on site up to ten days. A final report could take up to two years.