Natural gas system built of aging pipes

Thursday’s explosions in Massachusetts shed light on the condition of the 2.5 million-mile fuel network.

It is now safe for thousands of Massachusetts residents to return to their homes, 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 Sunday that police, firefighters and utility crews had “safely cleared” of gas thousands of homes in Andover, North Andover and Lawrence.

Authorities have not, however, explained exactly what caused the explosions, and a spokesman from the Massachusetts Emergency Management Agency did not immediately answer questions Sunday.

The gas lines of Columbia Gas of Massachusetts have been the focus of investigators. Authorities have previously said they were looking at the possibility that gas may have been placed under a level of pressure that was too high for the pipelines it was moving through.

The announcement Sunday follows the completion of what authorities are calling Phase 1 of the response to the explosions.

In the next phase, authorities will examine the damage to the gas system, and after that, they will inspect “every piece of equipment” involved, Baker tweeted.