LAWRENCE, Mass. (AP) - Federal officials say the investigation into the Boston-area natural gas explosions is partially focused on pressure sensors that were connected to a gas line that was being taken out of service shortly before the blasts.

National Transportation Safety Board Chairman Robert Sumwalt said Sunday that the sensors can signal for gas pressure to be increased if the pressure gets too low. He said investigators will try to determine whether those sensors played any role in Thursday's explosions and fires.

Dozens of homes were destroyed or damaged, a teenager was killed and dozens of people were injured in Lawrence, North Andover and Andover.

Officials say it appears there was too much pressure in Columbia Gas' pipelines. The gas company also is investigating.

Gas was turned off and residents were allowed to return to their homes Sunday.