Correction: An earlier version of this story incorrectly referred to the fertilizer facility as a plant. It is a retail facility, and the story has been corrected.

— Rescue crews are searching the smoldering remains of a fertilizer facility in central Texas, and a swath of destroyed homes around it, for victims of a massive explosion that injured more than 160 people and may have killed as many as 15, authorities said.

The estimate of how many may have died in the explosion Wednesday night includes an estimated three or four “first responders” who remained missing hours after the blast, most likely volunteer firefighters or ambulance workers. Officials said they were among many who responded to a fire at the West Fertilizer Co. retail facility and were urgently trying to evacuate people from the area, after realizing that the blaze had the potential to trigger a huge blast.

“Last night was truly a nightmare scenario in that community,” Texas Gov. Rick Perry (R) said Thursday, adding, “This tragedy has most likely hit every family” in West, a tiny town 20 miles north of Waco with a population of 2,800.

(See the latest updates on the Texas explosion here.)

Officials said there was no immediate indication that the fire was anything other than an industrial accident, but added they were examining all possibilities. The federal Bureau of Alcohol, Tobacco, Firearms and Explosives and the U.S. Chemical Safety Board, a federal agency that investigates industrial disasters, said they were sending teams of investigators to West to assist local authorities.

“We are not ruling anything out,” said Waco police Sgt. Patrick Swanton, a police spokesman. “Until we know that it was an industrial accident, we will investigate it as a crime scene.

Courtney Adair, 20, a great-niece of the owner of the facility, said the family is in shock. “They don’t know what to think or what’s going to happen,” she said in an interview.

Adair said the family can’t fathom that anyone would have set the blaze on purpose, saying her great uncle has no enemies.

“Honestly, I think this was an accident,” she said. “West is a small country town.”

Residents in this rural town, which takes pride in its Czech roots, were dazed Thursday as the extent of the damage started to become clear. Wendy Maler, 37, who lives 750 feet from the facility, said her husband, a volunteer firefighter urged her and their kids to evacuate as the fire started spreading.

Barely five minutes after reaching her mother-in-law’s house, just down the road, the ground shook as a ball of fire shot into the sky.

“We just grabbed the kids and hit the ground,” she said. David Maler, 47, her husband, was wounded as he tried to help put out the fire.

Her home was wrecked, Maler said. Windows were blown out, the drywall crumbled, and the doors caved in.

“We were able to go into the house and get our wedding rings, but that was it,” she said.

Perry told reporters he was declaring McLennan County a disaster area. He said President Obama called him Thursday morning from Air Force One en route to Boston, where the president was to attend a prayer service for victims of Monday’s Boston Marathon bombings.

The fire at the fertilizer facility was reported at 7:29 p.m. Wednesday, Swanton said. The first report of an explosion came at 7:53 p.m.

West Mayor Tommy Muska said 50 to 60 homes within a five-block radius of the factory were heavily damaged. West Rest Haven Nursing Home was being evacuated at the time of the blast because of the facility’s proximity to the fire; its residents have all been brought to a safe location, Muska said.

“There are homes leveled. There are businesses leveled,” Swanton said in a news briefing. “There is massive devastation in the downtown West business area.”

The neighborhood around the facility has been evacuated, but rescue workers are searching house to house, looking for people who may have been injured, killed or trapped in the debris, Swanton said. The scores of people taken to area hospitals for treatment had injuries ranging from minor to critical, hospital officials said.

When West Fertilizer caught fire, Tedd Uptmore, the longtime sales manager, jumped in his vehicle and started heading in its direction. But police stopped the 80-year-old before he got there. “That probably saved his life,” said his son, John.

The son heard the later explosion at his house four miles outside of West. He rushed to the scene and saw the facility. “It’s flat now,” John Uptmore said in a telephone interview. He made his way through thick black smoke to help evacuate elderly residents from the nursing home. “I helped move them to the community center. It was a disaster. There was a lot of glass. People had abrasions all over from the glass.”

John Uptmore said he stayed past 10 p.m., about three hours, until officials told him plenty of professionals had arrived to help. Uptmore’s voice started to crack. He had visited the facility before the explosion, but would not describe how it looked then or its contents. He said his father wouldn’t comment. “He’s not talking. He can’t. He’s devastated. The owner has to make that statement,” John Uptmore said.

In a statement issued by the White House, Obama offered whatever federal assistance is needed “to make sure there are no unmet needs as search and rescue and response operations continue.”

“Today our prayers go out to the people of West, Texas,” Obama said. “A tight-knit community has been shaken, and good, hard-working people have lost their lives.”

Muska, who in addition to being mayor is also a firefighter, told reporters that he was heading toward the fire when the thundering explosion occurred. “It blew my hat off,” he said, looking startled. “It blew the rearview mirror off my truck. It was a very powerful explosion.”

The mayor said his town, home to 2,800 people, needs prayers. While specifics about who may have been killed remain unknown, he predicted solemnly Wednesday night, “There are a lot of people who aren’t gonna be here tomorrow.”

A relief center for evacuees has been set up at Abbott High School, Swanton said, and people whose loved ones are unaccounted for can go there to try to find them. Muska said Thursday morning that a hotline with information about victims would be activated as of 9 a.m. local time (10 a.m. Eastern). The number is 254-826-4115, he said.

The explosion “was massive, just like Iraq, just like the Murrah Building in Oklahoma,” D. L. Wilson, a spokesman for the Texas Department of Public Safety, told reporters, referring to the April 19, 1995, bombing of the federal building in Oklahoma City.

Authorities were struggling Thursday morning to get a clear sense of the damage in West, in part because the explosion disrupted power and cellphone service to parts of the area. Stanton said he believed all fires at the facility were under control, and the threat from potentially hazardous ammonia billowing through the air had dissipated.

“We’re going house to house, business to business,” Swanton said. “I think we’re going to see fatalities increase.” The explosion, he said, “reached blocks, if not miles, in its devastating effect.”

State officials “are monitoring developments and gathering information as details continue to emerge about this incident,” Perry said in a statement Wednesday night. “We have also mobilized state resources to help local authorities. Our thoughts and prayers are with the people of West, and the first responders on the scene.”

The Environmental Protection Agency fined the facility $2,300 in 2006 for having a deficient risk management plan, according to the agency.

The EPA “found a number of deficiencies” with the retail facility during its March 16, 2006, inspection, according to agency spokeswoman Alisha Johnson. They included a failure to update the plan, which was due two years earlier; a failure to address the hazards identified in the company’s safety review; poor employee training records; and the lack of a formal written maintenance program.

The federal government requires risk management plans – which outline how a facility reduces the chances of an accidental leak of extremely hazardous material and how it would respond to any hazardous release — for plants and facilities with significant amounts of dangerous chemicals.

In the case of West Fertilizer Co., Johnson wrote in an e-mail, the facility fell under the requirement because “the quantity of ammonia on-site exceeds 10,000 lbs.” She added that the company “has not had a major accident in the last five-years.”

The blast came on a grim anniversary for the Waco area. Twenty years ago this week, 76 members of the fringe Branch Davidians religious group were killed after setting fire to their building when federal agents attempted to serve a search warrant.

Darryl Fears and Juliet Eilperin contributed to this story.