Rescuers in Texas on Thursday carried out a painstaking search for survivors after a massive explosion at a fertilizer factory killed as many as 15 people and destroyed dozens of homes.
With the country already on edge after the deadly Boston Marathon attacks, the factory in the small town of West blew up in a huge fireball Wednesday, an explosion so great experts said it had the force of a 2.1-magnitude earthquake.
Authorities said they feared they could find more bodies in the rubble of homes and businesses leveled by the tremendous explosion, which was preceded by a fire at the West Fertilizer plant in the southern US state.
A huge part of West was evacuated overnight, and Texas Governor Rick Perry said Thursday that local schools would remain shuttered for the remainder of the week.
"Last night was truly a nightmare scenario for that community," Perry told a press conference in the state capital Austin, announcing that he was seeking a federal disaster declaration which would make additional funds available.
"This tragedy has most likely hit every family, it has touched practically everyone in that town," Perry said.
Police sergeant W. Patrick Swanton of the department in nearby Waco said the tragedy killed "anywhere from five to 15" people, but warned he expected that toll to rise. Hospitals here have treated more than 160 people with varying injuries, he said.
Swanton told reporters at a briefing that officials were conducting a massive door-to-door search and rescue operation "to still find survivors, to still find people that are injured."
Officials said they do not yet know what caused the explosion, but are treating the factory site as a crime scene until the investigation rules out foul play.
"We are not indicating that it is a crime, but we don't know," Swanton said overnight.
Others however said the main cause could be anhydrous ammonia, a chemical used in the production of fertilizer. Authorities said the investigation was being led by the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.
The blast, which was felt as many as 50 miles (80 kilometers) away,
destroyed an apartment complex and a nursing home and sent residents fleeing into emergency shelters.
An expert at the US Geological Survey told AFP the force of the explosion had registered as a 2.1-magnitude seismic event.
President Barack Obama offered the prayers of the nation to the people of West, a town of about 2,800 people, which is home to a thriving Czech community dating back to the late 1800s when immigrants settled the American frontier.
"A tight-knit community has been shaken, and good, hard-working people have lost their lives," he said, offering the assistance of the Federal Emergency Management Agency.
The US Army national guard dispatched a 21-member "civil support team" to monitor air quality for possible hazardous emissions, a spokesman said, amid worries that toxic fumes could sicken residents of West and nearby areas.
The explosion at the West Fertilizer Company came with the nation still raw with emotion after the Boston marathon bombings Monday, which left three people dead.
Americans also were on edge after letters apparently laced with the deadly poison ricin were sent to Obama and a US senator in Washington. Police arrested a suspect in that case late Wednesday.
Search and rescue efforts in West could be complicated by a storm system heading into the area, with forecasters predicting heavy rains, and winds possibly heavy enough to spawn tornadoes.
In Prague, the foreign ministry said its ambassador to Washington, Petr Gandalovic, was traveling to West to study the possibility of providing aid from his government for the injured and relatives of the victims.
The Federal Aviation Administration declared a no-fly zone over the blast zone, over fears another explosion could bring down small aircraft.
Witnesses said they were stunned by the sheer force of the blast.
"It knocked me down, it knocked me back. It was like the whole road just picked up," resident Cheryl Marich, whose home was destroyed, told CNN.
The explosion came just ahead of the 20th anniversary on Friday of a deadly confrontation in Waco between federal authorities and heavily armed members of the Branch Davidian religious group, stoking worries about possible foul play.
In the 1993 Waco siege, following a 51-day standoff, the group's compound burned down after an assault was launched.