It is now safe for thousands of Massachusetts residents to return to their homes, the authorities said, three days after gas leaks touched off a series of surprise explosions and fires that killed one and injured more than 20 others.

Thousands had evacuated after a series of violent explosions Thursday afternoon. Gov. Charlie Baker of Massachusetts tweeted on Sunday that the police, firefighters and utility crews had “safely cleared” of gas thousands of homes in Andover, North Andover and Lawrence.

The authorities have not, however, explained exactly what caused the explosions, and a spokesman from the Massachusetts Emergency Management Agency did not immediately answer questions on Sunday. The National Transportation Safety Board is investigating but did not immediately return requests for comment on Sunday.

The gas lines of Columbia Gas of Massachusetts, which is part of the utility company NiSource, have been the focus of investigators.