(CNN) The National Transportation Safety Board will investigate a pressure increase detected in a natural gas pipeline north of Boston as it tries to figure out what caused a series of fires and gas explosions in three Massachusetts towns, NTSB Chairman Robert Sumwalt said Saturday at a news conference.

A pipeline controller working at a console in Columbus, Ohio, noticed the pressure increase in a pipeline in Lawrence, Massachusetts, one of the three towns where the fires occurred, Sumwalt told CNN's Alison Kosik. He didn't say when the pressure increase was noticed.

"Certainly we will interview that pipeline controller," he said.

Also Saturday, Massachusetts Gov. Charlie Baker said most people who had to evacuate their homes in Lawrence, Andover and North Andover should be able to go home Sunday morning.

"Over 5,000 customers have had their power restored. We expect to have the vast majority of the remaining homes done and cleared for gas tonight," Baker said. "People, unless something unanticipated happens, should be able to return to their homes at 7 a.m. tomorrow morning."

Thousands had to evacuate after more than 60 suspected gas fires broke out Thursday evening in homes in the towns north of Boston. One person was killed, several were hurt and dozens of people were evacuated. About 18,000 customers lost power at one point, National Grid reported.

The NTSB, which is investigating because transportation -- in this case, of natural gas -- was involved, arrived in Massachusetts on Friday morning to investigate the fires and gas explosions and will also be looking into the system procedures, operations, and safety culture of Columbia Gas and its parent company NiSource, as well as constructing a timeline of events surrounding the fires.

"We are very interested to understand the operations of Columbia Gas. We will be looking at their record keeping, their procedural compliance, their procedures, their training, the oversight of their contractors," Sumwalt said Saturday.

Sumwalt also noted there are 14 gas pressure regulators in the area and that investigators plan to test all of them and make sure they are all operating properly. He expects that NTSB investigators will remain on scene for seven to 10 days while they collect perishable evidence and conduct interviews.

"Our mission is to find out what happened so we can keep it from happening again," Sumwalt said.

Sumwalt said they will work to determine a cause for the incident and issue a preliminary report, but that a full detailed report on the exact cause may not be complete for up to two years.

There is no evidence at this time that anything nefarious contributed to the fires, Sumwalt said.

State of emergency

Speaking at a news conference Saturday night, Baker said their recovery has three phases:

• Getting people back into their homes safely and restoring electricity.

• Assessing the damage to the low-pressure gas system in the affected area.

• Having technicians and other experts inspect every piece of equipment that distributes gas.

He urged people returning home to inspect all their appliances that use natural gas and to call authorities if something seems wrong.

Gas mains were shut down in the three towns. Hundreds of natural gas technicians were to descend on the towns in the coming days to restore gas service, state officials said.

Words are not enough to thank everyone that assisted with much needed equipment and supplies during this difficult time. A special THANK YOU also to all our partners, DPWs and DOT across the region who also helped. #PowerOfPublicWorks #LawrenceStrong pic.twitter.com/n9stuWnFUA — Lawrence DPW (@Lawrence_DPW) September 15, 2018

Jeff Hall, spokesman for the Red Cross, said Friday there were almost 400 people in shelters after the Thursday night explosions and fires.

"The evacuations are in the thousands," Hall said Friday, adding he expects many residents were staying with friends and family in the surrounding area.

Lawrence Mayor Dan Rivera said there are about a dozen families who are "permanently displaced."

Baker declared a state of emergency Friday in the three towns and put another utility company in charge of restoration efforts. Baker said New England-based Eversource would replace Columbia Gas of Massachusetts

Eversource tweeted Saturday that it was working to shut off more than 8,000 gas meters, and the company said it anticipated "this critical first phase will be substantially complete" Saturday night.

A house goes up in flames Thursday in northeastern Massachusetts north of Boston.

Friday, the governor said Columbia was "simply inadequately prepared" to effectively manage relief efforts.

Rivera said of the company, "It just seemed like there's no one in charge, like they're in the weeds, and they've never seen this before."

Utility President Steve Bryant later defended the company's performance, saying the company had remained in constant touch with state and federal officials.

"I would say that we've advanced this as rapidly as it could possibly be advanced," he told reporters. "I don't think that anybody else managing this would've been further down the road than we are at the moment."

Bryant expressed his condolences to the victim's family, apologized for the inconvenience to customers and said the company was providing "all the resources that we possibly can to be able to correct the damage."

It 'looked like Armageddon'

Thursday evening, as homes erupted in flames from suspected gas fires, Andover Fire Rescue Chief Michael B. Mansfield said the scene "looked like Armageddon."

One house in Lawrence exploded, sending the chimney flying. It crashed into a nearby car, fatally wounding a teenager who was inside. Leonel Rondon, 18, and two of his friends were in the car when the debris struck, his family told CNN affiliate WHDH . He later died at the hospital, authorities said.

As pressure mounted for gas company officials to provide answers about what happened, Sens. Ed Markey and Elizabeth Warren of Massachusetts called for a congressional hearings on the explosions and fires.

Firefighters battle a house fire Thursday in North Andover, Massachusetts.

Utility works with officials to 'investigate this incident'

A house in Lawrence is reduced mostly to rubble Thursday after a series of explosions.

This is not the first time that a subsidiary of NiSource, the parent company of Columbia Gas of Massachusetts, has been the subject of a federal probe stemming from a disaster.

A Columbia Gas Transmission Corporation natural gas pipeline burst in West Virginia in 2012 and NTSB investigators determined that external corrosion, which could have been discovered by the pipeline operator, was at fault.

Columbia Gas Transmission was owned by NiSource until 2015, when the company was spun off into Columbia Pipeline Group. Later, that company became part of TransCanada.

NiSource is one of the largest natural gas utilities in the United States, serving more than 3.4 million customers in seven states, according to its website. The company also provides electric distribution, generation and transmission services to almost 500,000 customers in northern Indiana as well.