(Adds Obama's emergency declaration, paragraphs 14-15)
By Corrie MacLaggan and Lisa Maria Garza
WEST, Texas, April 19 (Reuters) - Investigators sifted through debris on Friday to pinpoint the cause of a Texas fertilizer plant explosion that obliterated parts of a small town and killed at least 14 people, including volunteer firefighters who raced to the scene to douse a blaze.
There was no indication of foul play in the fire or the blast it triggered on Wednesday at West Fertilizer Co, a privately owned retail facility that was last inspected two years ago, authorities said.
The farm supply business, located at the edge of a residential area in West, a town about 80 miles (130 km) south of Dallas, had notified a state agency that it stored potentially combustible ammonium nitrate on the site.
Mayor Tommy Muska told a news conference the confirmed death toll had risen to 14, based on the number of victims whose remains had been recovered from the vicinity of the blast. Authorities said 200 people were injured.
Among the dead were five volunteer firefighters from the town of West, as well as four paramedics, a retired firefighter who was assisting the volunteer squad and a Dallas fire captain who lived in West and responded to the scene, the mayor said.
U.S. Senator John Cornyn of Texas said the town's deputy fire marshal told him that 60 people remained unaccounted for. But he said that number was expected to drop as individuals turn up at area hospitals or with relatives and others, some of them outside of town.
"I would just take that (number) with a grain of caution," Cornyn said.
The majority of the confirmed dead were emergency personnel who responded to a fire and likely were killed by the ensuing blast, which was so powerful it registered as a magnitude 2.1 earthquake.
It left a devastated landscape, gutting a 50-unit apartment complex, demolishing about 50 houses and battering a nursing home and schools. Dozens more homes were reported to have been damaged.
The ruins of nearly 175 homes and other buildings left badly damaged or destroyed had been searched and "cleared" as of Friday afternoon as rescue teams combed wrecked structures for people who might have been trapped, officials said.
After touring the scene on Friday, Governor Rick Perry told reporters he was advised that "the search and rescue phase is complete." Asked whether that meant no more survivors were expected to be found, he said he did not know enough to comment.
The precise origins of the disaster remained a mystery as agents from the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives began investigating the blast site on Friday to collect debris and other evidence that may point to a cause.
The explosion was one of a series of events that put Americans on edge this week, starting with the Boston Marathon bombings and the discovery of poisoned letters addressed to President Barack Obama and a Republican senator.
Obama issued an emergency declaration for Texas on Friday authorizing the Federal Emergency Management Agency to mobilize any resources it needed necessary to help the state cope with the aftermath of the fertilizer plant blast.
"I want them to know that they are not forgotten," Obama said in an appearance in the White House briefing room. "All in all, this has been a tough week."
Authorities said they were treating the blast site as a crime scene, although they said they strongly suspected an industrial accident.
The death toll was huge for a town of 2,800 residents, and everyone seemed to know someone who died or was presumed dead.
Brian Uptmor, 37, said his brother disappeared after he went toward the fire on Wednesday night to try to save horses in a pasture near the plant.
William "Buck" Uptmor, 44, has not been found among the injured at area hospitals, has not answered his cell phone and his truck has not moved from where he left it.
"He is dead. We don't know where his body is," said Uptmor, a former firefighter. "It'll probably hit me at the funeral."
Hundreds of mourners in town, known for its ethnic Czech heritage, packed St. Mary's Catholic Church of the Assumption for a remembrance service on Friday evening honoring the firefighters and others killed in the blast.
Surviving members of the volunteer fire department, all dressed in red T-shirts, exited the service together in a grim procession and gathered outside around one of their fire trucks.
One firefighter injured at the blast site used crutches, most of his right arm bandaged. After he was helped into a passenger seat of a waiting SUV, a woman got out of her car and leaned into his window to hug and kiss him.
"I love you," she said through tears. "I love you so much." The firefighter wiped his face as he rolled up the window and was driven away.
West Fertilizer Co blends fertilizer and sells anhydrous ammonia and other chemical products to local farmers. It stored 270 tons of "extremely hazardous" ammonium nitrate, according to a report filed by the company with the state.
Farmers use anhydrous ammonia as fertilizer to boost soil nitrogen levels and improve crop production.
The West plant is one of thousands of sites across rural America that store and sell hazardous materials such as chemicals and fertilizer for agricultural use. Many are near residences and schools.
The plant was last inspected for safety in 2011, according to a Risk Management Plan filed with the federal Environmental Protection Agency.
The company, which has fewer than 10 employees, had provided no contingency plan to the EPA for a major explosion or fire at the site. It told the EPA in 2011 that a typical emergency scenario at the facility that holds anhydrous ammonia could result in a small release in gas form.
The EPA fined the firm $2,300 in 2006 for failing to implement a risk management plan.
Donald Adair, a lifelong resident of West and owner of the plant's parent company, Adair Grain Inc., issued a statement on Friday saying, "My heart is broken with grief for the tragic losses to so many families in our community."
He added that his company was "working closely with investigative agencies" and pledged "to do everything we can to understand what happened to ensure nothing like this ever happens again in any community."
While authorities stressed it was too early to speculate on the cause of the blast, a forensic sciences expert said investigators probably would consider at least two scenarios.
John Goodpaster, assistant professor and director of forensic sciences at Indiana University-Purdue University Indianapolis, said anhydrous ammonia was stored in liquid form but formed a vapor when mixed with air that can be explosive.
If enough heat is applied to a container of anhydrous ammonia, he said, "that container could become a bomb."
A second possibility is that ammonium nitrate could have exploded, said Goodpaster. That was the cause of one of America's worst industrial accidents, the 1947 explosion of a ship in a Texas City port that killed nearly 600 people. (Additional reporting by Colleen Jenkins, Carey Gillam, Nick Carey, Anna Driver and Josh Schneyer; Writing by Greg McCune and Steve Gorman; Editing by Xavier Briand and Peter Cooney)