The blasts, which led to scores of simultaneous structure fires across three towns in the Merrimack Valley, filled otherwise sunny skies with thick smoke and pushed thousands of residents out of their homes indefinitely. Electrical power has been cut to the communities, and residents have been told not to enter their homes until each one has been inspected for potential dangers.

Columbia Gas of Massachusetts, which owns the gas lines involved in the blasts, has thus far given no indication of what might have caused the disaster. Baker and other officials, including Lawrence Mayor Dan Rivera, issued scathing criticisms of the company.

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“Since yesterday, when we first got word of this incident, the least informed and the last to act have been Columbia Gas,” Rivera said, with Baker at his side at a news conference. He said that the company had promised “hundreds of teams of technicians” but that “none have materialized.”

“It just seems that there’s no one in charge,” Rivera said. “Like they’re in the weeds.”

At a news conference a couple of hours later, Columbia Gas President Steve Bryant defended the company’s response. “We advanced this as rapidly as it could possibly be advanced,” he said. “I don’t think that anybody else managing this would have been further down the road then we are at the moment.”

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Bryant said the company had nearly 300 technicians in the field who had turned off gas to more than 3,200 of the affected customers, a necessary step before electricity can be restored. He said gas would be shut off to all homes in the area by Saturday or Sunday, allowing power to be turned back on and people to move back in.

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He referred questions about the cause of the incident to the National Transportation Safety Board (NTSB), which is leading the investigation.

“We are sorry and deeply concerned about the inconvenience,” he said.

Dazed and nervous people stood in the streets or milled around shelters as state and federal officials and Columbia Gas technicians methodically inspected buildings in Andover, North Andover and Lawrence, just north of Boston.

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Investigators had begun to zero in on the potential cause of the explosions: over-pressurization of a gas main owned by Columbia Gas, which had been upgrading equipment in the area, according to Jennifer Mieth, a spokeswoman for the state fire marshal’s office.

Before the explosions on Thursday, Columbia Gas notified customers that it would be “upgrading natural gas lines in neighborhoods across the state.” The utility said the move would bring increased reliability and “enhanced safety features.”

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NTSB Chairman Robert Sumwalt said Friday that his agency, which handles incidents involving pipelines, is “looking at the design of the pipeline system, any maintenance or upgrades that are in the process of being done.”

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On the streets here Friday, residents spoke of loss, confusion and disbelief that something so catastrophic could result from something as commonplace as natural gas supply pipes — natural gas supplies energy to more than 68 million homes nationwide, according to the U.S. Energy Information Administration.

Leonel Rondon, 18, of Lawrence, was killed when an explosion at a house toppled a chimney onto the car in which he was sitting.

His father, Miguel Rondon, stood outside his rowhouse Friday watching a family friend who had volunteered to talk with reporters about his son as other relatives gathered on the steps. Miguel Rondon said the last time he saw his son was Thursday, when he showed up after taking his driving test.

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“He was happy, happy,” Miguel Rondon said. “He made his license.”

Hector Rondon, a relative of Leonel Rondon, handed a single sheet of paper to waiting reporters, with a short description, carefully prepared in English, of the young man the family had just lost.

“Excited about life,” it read. “Loving and respectful.”

And below, in tidy green script: “He liked cars.”

Hector Rondon took the piece of paper back, folded it in half and then in half again and turned away, briefly raising his hands to his red, watering eyes.

Bryant, the Columbia Gas president, said the company extended “deepest condolences” to Rondon’s family.

But officials sounded increasingly frustrated and furious as the day wore on.

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Rivera said he thought Columbia Gas was “hiding from the problem.”

“It is in the best interest of the people of Lawrence, North Andover and Andover for us to get another team leading this,” Baker said at the news conference with Rivera. With his emergency declaration, Baker said he was instructing the Department of Public Utilities to authorize Eversource, another private energy company, to take management control over the effort to restore utility services.

Baker, Rivera and the state’s two U.S. senators, Democrats Elizabeth Warren and Edward J. Markey, toured Springfield Street on Friday. Rep. Seth Moulton (D), who represents the area in Congress, also visited the neighborhood.

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“I think a lot more people would have died if our first responders hadn’t been on the scene so quickly and so effectively,” Moulton said in an interview. “The gas company took five hours to let anyone know there was even a problem — and even then they didn’t call for people to evacuate. I believe the evacuation saved lives. And the evacuation wasn’t driven by the gas company; it was driven by local first responders.”

Moulton said “the entire bipartisan political leadership of the state” is “going to make sure that this gas company is held accountable.”

Bryant said the company has a “tremendous track record” and responded as quickly as possible.

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In the streets, people forced from their homes walked in small groups of three or four, carrying backpacks, pulling roller bags and towing wagons overflowing with possessions stuffed in plastic bags — Social Security cards, birth certificates, medications.

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Neighbors remained anxious about leaving their homes unattended. Standing mostly outside to obey evacuation orders, members of one extended family guarded their homes and their block.

Justina Lebron and her family spent one night in a hotel and plan to move in with her mother. They have milk and ham and other groceries in their wagon, and for now, they aren’t concerned about leaving an empty house. Thursday night, the helicopters overhead worried Lebron.

“I thought it was a crime,” she said. But since then, there has been a parade of patrol cars through the neighborhood. What worries the family more is what comes next.

“What is the next step?” she asked. “That’s what we don’t know.”

Authorities hope to be able to get people back into their homes by Sunday, but they are going from house to house to check on gas connections, an effort that has been made more complex because of mandatory evacuation orders in some areas — meaning people aren’t home to let the gas company in.

Officials and experts cautioned that the cause of the blasts was still unknown, but they were focusing on “over-pressurization” as the most likely explanation.

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Natural gas is distributed through a network of 2.5 million miles of underground pipes that carry it from its source to homes and businesses across the country, according to the Pipeline and Hazardous Materials Safety Administration (PHMSA), which sent investigators to Massachusetts.

According to PHMSA statistics, between 1998 and 2017 there were 745 “serious” incidents involving natural gas pipelines, resulting in 278 deaths and 1,153 injuries. In Massachusetts in that same period, there were 15 serious incidents resulting in eight deaths and 31 injuries. Before Thursday, there had been none since 2012.

Michael Ahern, director of power systems engineering at Worcester Polytechnic Institute, said that once gas is extracted from the earth, it is carried first in large pipes that act as a sort of “interstate highway system” for gas transmission.

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Gas at high pressure, generally about 1,000 pounds per square inch (psi), is carried long distances in steel pipes with a diameter of 18 to 36 inches, Ahern said. In the case of Massachusetts, those lines will arrive from gas sources in Texas, Pennsylvania or Canada, he said.

Those large transmission lines then arrive at regional “gate stations,” where pressure is reduced to an average of 10 to 50 psi, and then distributed locally in pipes that are usually about 8 to 12 inches in diameter, Ahern said. Those smaller pipes can be cast iron or steel if they are older, while newer pipes are plastic, he said.

When those pipes arrive at homes and businesses, the pressure is reduced again, with regulators dropping the pressure to perhaps 1 psi or less and directing the gas through copper or plastic pipes less than an inch in diameter that feed stoves, water heaters and other appliances.

Ahern said gas has been safely delivered this way for more than a century. While he was unaware of the specifics of Thursday’s explosions and fires in Massachusetts, he said he suspected there was a “unique set of circumstances” that caused it, without reason to worry about the overall safety of gas delivery generally.

He said instances of systems failing because of over-pressurization are rare. The most recent he could recall was in San Bruno, Calif., in 2010, when eight people were killed when a large gas transmission line exploded. Over-pressurizing gas lines can result in too much gas being forced into homes, creating a situation in which a pilot light or any other source of flame can ignite it and cause an explosion and fire.

Two cases of gas leaks in residences led to deaths in the Washington region within the past 20 years. In 1998, gas collecting in the basement of a new Loudoun County home ignited when a pilot light sparked, killing a 40-year-old mother of two, critically injuring her husband and launching their two children out the front windows of their second-floor bedroom. In 2016, natural gas built up in the basement of a Silver Spring apartment complex and exploded, killing seven people and injuring more than 30.

Standing outside her still-smoldering Springfield Street home in Lawrence on Friday, Jenny Caceres, 36, said that on Thursday night she was trying to scramble eggs for her two daughters but her stove burners sputtered and wouldn’t stay lit.

She called her landlord, who went to the basement and found signs of trouble. A gas alarm was going off, but the meter wasn’t running. He immediately banged on Caceres’s door and told her to get out of the second-floor apartment. She stood across the street with her daughters, with only the clothes they were wearing. Her 12-year-old didn’t have shoes on.

“We just ran out. I didn’t want to risk staying in there and something happening to my daughters,” said Caceres, a Spanish-

language translator for the Lawrence school district.

The building caught fire after she escaped. She doesn’t know the condition of the apartment or the two birds she was pet-sitting for her grandmother.

“I have no idea,” she said through tears.