As residents of Lawrence, Andover and North Andover are cleared to return home Sunday morning, federal investigators determine there's no evidence to suggest the gas explosions that rocked communities north of Boston were intentional.

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. Charlie Baker declared a state of emergency for the area and put Eversource Energy, another utility, in charge of recovery efforts after state and local officials said they were unsatisfied with Columbia Gas' response.

Company president Stephen Bryant defended his company's performance Friday, saying it had more than 300 workers and contractors responding, with another 100 on the way.

Meanwhile, a report of a strong gas odor brought officials back to a part of Lawrence most impacted by the explosions and fires Saturday morning.

Firefighters and gas workers opened manholes, tested gas levels and tried to gain access to businesses on the city's south side, but the situation appeared to be contained by 11:15 a.m.

Local officials also warned about donation scammers.

They said some residents in Lawrence and Andover have received calls from individuals falsely claiming they're collecting donations on behalf of a local firefighter's association for the disaster.