LAWRENCE, Mass. (AP) — There’s no evidence to suggest the gas explosions that rocked communities north of Boston were intentional, a federal investigator said Saturday. As federal officials opened their investigation into Thursday’s disaster, National Transportation Safety Board Chairman Robert Sumwalt said that there doesn’t appear to be “anything nefarious, anything suspicious, anything intentional.” He said investigators will be working to develop a timeline going back at least three weeks, including reviewing any reports of gas odors reported by residents in the impacted communities of Lawrence, North Andover and Andover. Investigators will also look at how local Columbia Gas officials responded to a “pressure increase” in the Lawrence area that was detected at the company’s pipeline control center in Columbus, Ohio, prior to the explosions and fires. “We’re very interested to understand the operations of Columbia Gas,” he said. Sumwalt said NTSB investigators expect to be on site up to ten days but a final report about what happened could take up to two years to complete. The dramatic series of explosions and fires Thursday destroyed or damaged dozens of homes, killed a teenager, injured dozens and forced thousands of people to evacuate from the three Merrimack Valley communities. Massachusetts Gov.