WEST, Texas - Texas Gov. Rick Perry is calling the deadly fertilizer plant explosion in his state "a truly nightmare scenario."
Police say it's not clear how many people remain trapped in the rubble after the fertilizer plant explosion in the farming community of West that killed as many as 15 people and injured more than 160 others.
Perry emphasized during a Thursday morning news conference that much of the information about victims remains "very preliminary." He says President Barack Obama has offered a quick turnaround of declaring McLennan County an emergency disaster that is eligible for federal aid.
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Waco Police Sgt. William Patrick Swanton says early Thursday he doesn't know how many people have been rescued, but he says officials on the ground remain in "search and rescue mode," going building to building in the largely decimated neighborhood surrounding the plant.
He says he knows some firefighters still are missing.
President Obama called Texas Gov. Rick Perry on his way to an interfaith service honoring the victims of the terror attack at the Boston Marathon.
"We greatly appreciate his call and his gracious offer of support," Perry said.
Perry said he was declaring the entire county a disaster and will be requesting an emergency declaration from the president
Earlier Thursday, West Mayor Tommy Muska said more than 160 people were taken to area hospitals and 50 to 60 nearby homes were badly damaged.
Firefighters were initially called out to the plant on reports of a fire around 7:30 p.m. Some witnesses say the explosion that followed was so powerful, they initially thought it was an earthquake. The explosion could be heard as far away as Waxahachie, which is 45 miles away.
All residents of a nearby nursing home, who were evacuating because of the fire when the plant exploded, were able to make it out safely, Muska said.
Muska, who is also a firefighter, said he was a block away when the explosion occurred, and it knocked his helmet off and blew out the rearview mirror in his vehicle.
Photos on social media and chopper video showed people getting treated for injuries on a nearby football field, but that triage area was moved to a community center for safety reasons.
Swanton says there is no indication the blast was anything other than an industrial accident.
Red Cross spokeswoman Anita Foster says her group is working with emergency management officials in the town of West to find a safe shelter for residents displaced from their homes. She says teams from Austin to Dallas and elsewhere are being sent to the town of West, which is about 20 miles north of Waco.
The explosion has also leveled buildings for blocks in every direction. Al Vanek, a city council member, says a four-block area around the explosion "is totally decimated."
Some of the damaged buildings are a housing complex with a collapsed roof, a nearby middle school and the West Rest Haven Nursing Home.
The Occupational Safety and Health Administration most recently inspected the fertilizer plant in 1985.
Records reviewed by The Associated Press show that OSHA issued the West Chemical & Fertilizer Co., as the plant was called at the time, a $30 fine for a serious violation for storage of anhydrous ammonia.
OSHA cited the plant for four other serious violations of respiratory protection standards but did not issue fines. The maximum fine for a serious violation was $1,000.
It is not unusual for companies to negotiate lower fines.
OSHA has jurisdiction over more than 7 million workplaces. It's not uncommon for some companies to go years without inspection.
The Associated Press contributed to this report.