Volunteers unload a box truck at the senior center in Lawrence, Mass., on Saturday, delivering donations to help victims of the natural gas leak that sparked dozens of house fires in the area on Thursday. (Gabe Souza)

In a town that has struggled with social ills such as opioid addiction, gangs and crime, a place that long has hoped to recover from the loss of industry, the man-made calamity that crept in underground and delivered fiery terror this week added another layer of devastating challenges.

“It’s a tough city, a poverty city,” said a teary Zulayka Ovalles, as she stood in line for donations at a senior center. “A lot of us are just trying to make ends meet. And now? I have four children. It’s very hard to tell them we are homeless.”

Eighty natural gas explosions and fires on Thursday night across the Merrimack Valley here, north of Boston, left 8,600 homes in the area without gas or power, including large swaths of South Lawrence. Many have been forced into temporary homelessness, with some wandering darkened streets or standing sentinel outside their homes amid fears of looting.

Officials said Saturday night that residents of the three affected towns can return to their properties at 7 a.m. Sunday, even as the threat of additional leaks appeared not to have abated as the weekend began. Firefighters on Saturday were investigating two natural gas leaks just blocks from where several houses burst into flames a few days ago. Officials said each house must be inspected before power can be restored, as any gas leaks or buildup of gas inside homes could prove deadly.

One Lawrence teenager died when a chimney exploded off a house, the bricks landing on the SUV in which he was sitting.

Some worry that the gas company — Columbia Gas of Massachusetts — had not taken longtime complaints of gas smells seriously, and politicians accused the company of being slow to respond to the disaster, though the company says it has devoted all available resources to the response.

Deborah and Luis Rivera, now sleeping on her sister’s sofa, complained about the gas company as they stood in line at the senior center to pick up supplies. “I’m very, very mad,” Deborah Rivera said, explaining that she had called multiple times to complain about the smell of gas around her house. “They say it is normal.”

The gas leak appears to have been caused by a system put under too much pressure, causing gas to flow into houses, some of which caught fire. The system is run by Columbia Gas, the largest employer in Lawrence. On Friday, Gov. Charlie Baker (R) declared a state of emergency so he could shift control of the response to another company, Eversource.

The National Transportation Safety Board (NTSB) is investigating, and Chairman Robert Sumwalt said Saturday that officials will look at physical evidence, including 14 underground gas pressure regulators in the area, during the next week to 10 days.

“We have no evidence at this time that there is anything nefarious, anything suspicious, anything intentional associated with this disaster,” Sumwalt said.

He said an increase in natural gas pressure was detected at a Columbia Gas monitoring facility in Columbus, Ohio, about the time of the explosions, and the NTSB wants to understand what happened after the extra pressure was detected. They also will be examining complaints from customers during the past three weeks to see whether there was any recent increase.

“We will develop a complete timeline of the events surrounding this disaster,” Sumwalt said, later adding that he was personally “absolutely devastated” by the events. A complete investigation could take up to two years, he said, but he expects to establish the immediate cause much sooner.

Vivian Marquez, left, of Lawrence, Mass., comforts her neighbor, Lisa Rodriguez, as they wait in line for food, water and other supplies Saturday at a senior center. (Gabe Souza for The Washington Post)

In Andover, residents of most of the affected streets were allowed to return Saturday, and residents on nearly 40 streets were allowed to turn on their power by Saturday afternoon. Lawrence Mayor Dan Rivera had warned residents that it might take longer to turn on power in urban neighborhoods because of the higher density.

Officials began warning Saturday that it could be weeks before gas is restored to homes. Many people in the region start turning their heat on by the end of October, which could be a problem for those with gas heating systems.

Like many other once-thriving U.S. cities, Lawrence, population 80,000, lost the industries that powered this mill town and attracted people to work in the giant warehouses and factories lining the Merrimack River. Although New Balance shoes occupies a large facility, many remain vacant.

Always a hub for immigrants, including the Irish and Germans who arrived here in the mid-19th century to staff the woolen mills, Lawrence is now more than 75 percent Hispanic — a population of largely Dominican immigrants who began coming in the 1960s.

Many people living south of the Merrimack River speak little or no English.

Lesly Melendez, deputy director of Groundwork Lawrence, a social service organization, stood in front of the Lawrence Senior Center on Saturday morning, directing cars and people. Many were carrying sealed brown boxes of donated food and plastic bags of bedding. Volunteers showed others how to sign up to donate their time, as a line formed across the parking lot.

“People need baby formula, diapers and towels,” Melendez said. “That surprised me. Towels.”

Melendez, who grew up in Lawrence, said those who have resources have been incredibly generous to those without them. People are crashing on couches and floors across the city — as many as 32 in one woman’s apartment. It’s not unusual to find three generations of the same family now moving in together — often with in-laws north of the river who were not asked to evacuate.

If the explosions prove anything, Melendez said, breaking briefly to redirect a package of clothing, it’s that “Lawrence is resilient. We can come together.”

The Lawrence YMCA opened at 5 a.m. Saturday to offer free — but cold — showers to more than 200 people left temporarily homeless.

People lined up outside the senior center, clutching their uncharged cellphones, some staring in bewilderment at mountains of donations, unable to decide what they needed most when their needs were so great.

“We basically need everything,” said Liz Figueroa, beginning a list she did not know how to end. “Toothbrush, toothpaste … ”

A white flatbed truck set off across town, through the police barricades and over the Merrimack River, to deliver drinking water to the cordoned-off section of town where many residents are camping out.

“People look at us like we’re not worthy of anything, like we’re illiterate,” said Destiny Maldonado, referring to the people of Lawrence and the reputation for drugs and gangs. “This happens, and everybody comes together. It’s awesome. What can I tell you?”

Richard Gonzalez, 62, of Lawrence, Mass., refused to leave his home despite evacuation procedures. Gonzalez was sitting on his porch two days ago when he started to see smoke fill the sky after dozens of fires started in the area because of natural gas leaks. (Gabe Souza for The Washington Post)

The cheer among residents Friday, with pickup basketball games and shared cookouts from thawing freezers, was giving way to worries about looting.

“I am the only police in this house. I’ve got to stay,” one man called through an open window.

Though progress in Lawrence has been slow, some factories are being redeveloped for apartments and artist studios. Groundwork’s office is in a former mill building. “We are getting there,” Melendez said.

Lawrence and Lowell, which lies 10 miles upstream along the Merrimack River, have similar pasts: founded by early industrialists in the first half of the 19th century to make clothes. The textile business deserted Lowell sooner, but the city of 110,000 has successfully re-created itself as a hub for higher education and urban culture, becoming part of the “Massachusetts Miracle.”

Two years ago, Maine Gov. Paul LePage blamed people “from Lowell and Lawrence” for trafficking heroin and fentanyl across state lines, fueling the nation’s opioid epidemic.

In March, announcing plans to counter the opioid epidemic, President Trump, speaking in New Hampshire, called Lawrence a drug-ridden sanctuary city.

“Every day, sanctuary cities release illegal immigrants, drug dealers, traffickers, gang members into our cities,” Trump said. “They’re protected by these cities, and you say, ‘What are they doing?’ They’re safe havens for just some terrible people, and they’re making it very dangerous for our law enforcement officers.”

He cited a study in saying that Lawrence was “one of the primary sources of fentanyl in six New Hampshire counties.” Fentanyl, a powerful synthetic opioid, has been blamed for many opioid-related deaths.

Rivera was quick to fire back after Trump’s statement: “Shame on the president,” he said in a news conference. “He’s trafficking in pain and divisiveness, creating boogeymen where we need solutions.”

Thursday afternoon’s gas explosions reminded some residents of earlier fires in Lawrence, some the result of arson. In 1995, the Malden Mills factory was burned to the ground. The company’s owner famously paid employees — largely Lawrence residents — while the factory that made Polartec fleece was rebuilt. But the company, now under different ownership, announced three years ago that it would pull out of Lawrence.

Weintraub is a freelancer based in Massachusetts and is a frequent contributor to The Washington Post. Souza is a freelance photographer based in Maine.