— Rescue crews in this small town spent Thursday searching for victims in the smoldering ruins of a liquid fertilizer depot and dozens of destroyed homes after an explosion caused an unknown number of deaths and injured more than 160 people.

Authorities early in the day estimated that five to 15 people had been killed. But later, they declined to specify the number of people missing or dead.

A fire at the West Fertilizer Co. appears to have ruptured high-pressure storage tanks full of liquid ammonia, which caused the huge blast. Several firefighters were missing.

“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.”

West, with 2,800 residents, is a flat farming community 20 miles from Waco, just off an interstate highway, with a smattering of antique shops and locally owned diners. It plays up its Czech heritage, and like many rural towns, it has a demographic profile tilting to the elderly.

A day after the explosion, a large swath of the town was blocked to the public as emergency personnel continued to search for victims, picking through the rubble of collapsed homes and the flattened facility. The Red Cross set up a shelter at the town’s community center, which provided displaced families with basic supplies.

Officials said there was no indication the fire was anything other than an accident, but they said they were examining all possibilities.

“We are not ruling anything out,” said Waco Police Sgt. William Patrick Swanton, a police spokesman.

Mayor Tommy Muska said 50 to 60 homes within a five-block radius of the storage depot were heavily damaged. West Rest Haven Nursing Home was being evacuated at the time of the blast because of its closeness to the fire. Its residents were all brought to a safe location, the mayor said.

Muska, who 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.”

Authorities were struggling Thursday 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.

“We’re going house to house, business to business,” Swanton said at a news briefing. “I think we’re going to see fatalities increase.” The explosion, he said, “reached blocks, if not miles, in its devastating effect. There are homes leveled. There are businesses leveled.”

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. The federal government’s Bureau of Alcohol, Tobacco, Firearms and Explosives and the U.S. Chemical Safety Boar d, which investigates industrial disasters, were sending teams.

The cause of the fire is unknown. Courtney Adair, 20, a great-niece of the owner of the fertilizer distributor, said the family is in shock. “They don’t know what to think or what’s going to happen,” she said. She said the family can’t fathom that anyone would have set the blaze on purpose.

“Honestly, I think this was an accident,” she said.

Wendy Maler, 37, who lives 750 feet from the facility, said her husband, a volunteer firefighter, urged her and their children to evacuate as the fire started spreading. Barely five minutes after reaching her mother-in-law’s house 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.

Her 47-year-old husband, David, was injured fighting the fire. Their house was ruined — windows blown out, drywall crumbled, 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 headed to attend a prayer service for victims of Monday’s Boston Marathon bombings.

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.” The president added that “our prayers go out to the people of West” and that “good, hard-working people have lost their lives.”

Anhydrous ammonia has been used as a fertilizer since the 1940s and accounts for roughly one-third of the fertilizer used in this country. It is injected into the ground in liquid form. White cylinders containing the compound are a common feature of the rural American landscape.

Anhydrous ammonia burns, although not easily. However, if a storage cylinder melts in a fire and releases its contents, which then burn, the result can be catastrophic. The hazards of ammonia-filled tanks are well-known to firefighters.

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.

West Fertilizer did not make anhydrous ammonia. It stored and sold it. There are about 6,000 distributors like it around the country, said Kathy Mathers, spokeswoman for the Fertilizer Institute, which represents the industry.

After an inspection in September 2011, the federal government fined the company $10,100 for safety violations that included offering for sale and transport anhydrous ammonia “while failing to develop and adhere to a security plan,” according to documents from the U.S. Pipeline and Hazardous Materials Safety Administration.

The company showed it had properly labeled its cargo trucks, according to the documents, and also provided a cover page and table of contents for “their new security plan,” although that plan is not listed. The government then reduced the penalty to $5,250.

The Environmental Protection Agency fined West Fertilizer Co. $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, said EPA 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.

In the case of West Fertilizer, 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 same year that the EPA fined the facility, state regulators investigated and cited it after receiving complaints about its ammonia emissions, according to the Texas Commission on Environmental Quality.

“Ammonia Smell very bad last night from Fertilizer Plant, lingered until after they went to bed,” reads the June 6, 2006, report.

A nearby elementary school was evacuated in February because of a fire at the plant, according to a memo that the local school superintendent, Marty Crawford, sent to staff and parents.

Crawford wrote that the 911 dispatcher “did not acknowledge” that the retail facility “was carrying out a controlled burn of pallets and brush” and that “the district has asked emergency service providers for advanced notification in the future when the plant decided to conduct a burn.”

The memo did not say how the school, which serves fourth- and fifth-graders, learned or confirmed that the facility had been carrying out a “coordinated burn.”

The explosion 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.

Brown reported from Washington. Darryl Fears, Juliet Eilperin, Julie Tate and Ann Gerhart in Washington contributed to this report. Cody Permenter, a reporter for the Texas Tribune, contributed from West, Tex.