LAWRENCE (CBS/AP) — The investigation into the Merrimack Valley 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, the head of the National Transportation Safety Board said Sunday.

Related: North Andover Residents Frustrated By Lack Of Answers From Columbia Gas

NTSB Chairman Robert Sumwalt said 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.

“We can confirm this was an over pressurization situation,” Sumwalt said.

home1 NTSB Says Pressure Sensors Part Of Focus In Merrimack Valley Explosion Probe

A Lawrence home significantly damaged by a gas explosion. (WBZ-TV)

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

Read: Weeks To Fully Restore Gas Service After Merrimack Valley Explosions

Also Sunday, residents in Lawrence, Andover and North Andover were allowed to return to their homes after crews finished shutting off nearly 8,600 gas meters. Electricity was restored to all the affected homes and businesses, but gas service may not be restored for weeks.

Schools in Lawrence will be closed Monday because of the natural gas situation, officials said.

ntsb2 NTSB Says Pressure Sensors Part Of Focus In Merrimack Valley Explosion Probe

NTSB chairman Robert Sumwalt. (WBZ-TV)

Sumwalt said the investigation is focused on high pressure in pipelines. On Saturday, he said officials will be looking 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. He said there was no evidence the explosions were intentional.

Columbia Gas on Sunday turned away hundreds of Lawrence residents who wanted to make damage claims because it couldn’t handle the deluge, The Boston Globe reported. The company told residents, many of whom waited for several hours, to come back Monday.

Officials said gas company technicians will turn all the meters back after safety inspections of the entire system are complete — a process expected to take several weeks. They warned residents not to turn the meters back on themselves, not to turn on gas appliances until service is restored and to call 911 and leave their homes if they smell gas.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)

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