"It was a giant force of pressure just pushing me back," eyewitness Erik Perez recalled. "And then there was shrapnel flying everywhere from the explosion. I've been just shaken up since this whole thing happened. Probably worst thing I've ever seen in my life," he said.

Search and rescue teams are now going house to house hoping to find survivors. Several of the first responders, mostly volunteer firefighters, are among the missing.

Sharon Middlebrook's sister-in-law was in a nursing home near the plant.

"She said the ceiling collapsed on her, and when she got to the hospital and they removed the covers and things, she was covered with broken glass," Middlebrook said.

The city's mayor, Tommy Muska, told reporters the residents of West, about 20 miles north of Waco, needed their prayers.

"This is probably the major most devastating thing that's happened to this community," he said. "This is just a big old cut we got across our heart."

The explosion comes as the nation remains on edge after this week's bombings at the Boston Marathon.

It's still not known what caused the explosion, but investigators say they're treating the fertilizer plant as a crime scene until they can determine if it was an industrial accident.

The Bureau of Alcohol, Tobacco, Firearms and Explosives is taking the lead on the investigation into what caused the deadly blast.

"We're not indicating that it is a crime, but we don't know," Waco Police Sgt. William Patrick said. "What that means to us is that until we know that it was an industrial accident, we will work it as a crime scene. ATF is conducting the main investigation at that crime scene which is the fertilizer plant."

Meanwhile, Texas Gov. Rick Perry is calling the deadly fertilizer plant explosion in his state "a truly nightmare scenario." He emphasized during a news conference Thursday that much of the information about victims remains "very preliminary."

The governor said President Obama has declared McLennan County, where the explosion occurred, an emergency disaster that is eligible for federal aid.