Teams of first responders descended on the devastated town of West, Texas, early Thursday where a massive explosion at a fertilizer plant left scores of casualties and turned homes to rubble.

The number of dead remained unclear, with police saying it could be between five and 15. More than 160 people were injured and "three to four" firefighters were missing or unaccounted for, officials said.

Firefighters were battling the blaze that precipitated the explosion Wednesday night. And a storm system heading into the area brought helpful rain -- but also heavy winds that might make it much tougher to contain the fire.

It's unknown how many people may be trapped under rubble, authorities said early Thursday. Nails and other debris from destroyed buildings will pose safety risks for investigators as they comb through flattened areas, Waco Police Sgt. William Patrick Swanton said. Also, the Department of Homeland Security said federal and state authorities were taking steps to secure the area by shutting down local rail freight service and nearby utilities and restricting flights over the area.

Cause unknown

"Nothing at this point indicates we have had criminal activity, but we are not ruling that out," said Sgt. William Patrick Swanton of the nearby Waco Police Department. A U.S. intelligence official told CNN there is no indication so far that the blast is related to terrorism.

Most of the injured were hurt by the blast -- not by inhaling fumes, officials said. Many people had lacerations and puncture wounds.

Anhydrous ammonia, a gas used in making fertilizer, can cause severe burns if it combines with water in the body. Exposure to high concentrations can lead to death.

The West Fertilizer Co. said it had 54,000 pounds of the chemical, The Dallas Morning News reported.

There is no "chemical escape" that is "out of control," Swanton said.

There have been reports of "a small amount of looting," he said.

While Swanton said the death toll could be between five and 15, Dr. George Smith, the city's emergency management system director, said it could spike to 60 or 70.

"We have two EMS personnel that are dead for sure, and there may be three firefighters that are dead," Smith said.

"There are a lot of people that will not be here tomorrow," Mayor Tommy Muska warned late Wednesday.

About half the community was evacuated, Muska said, including a nursing home with 133 residents. A middle school is also located near the plant.

Depending how the winds shift, the other half of the town may have to be evacuated.

The Federal Bureau of Alcohol, Tobacco, Firearms, and Explosives has a team of 20 agents and forensic specialists assisting, a law enforcement source told CNN.

The Texas National Guard has sent 21 troops from a civil support team to monitor air quality near the blast, the Pentagon announced Thursday.

The White House said it is monitoring the situation through FEMA, which is in touch with state and local authorities. Federal authorities stand ready to help, a FEMA official said.

"A tight-knit community has been shaken, and good, hard-working people have lost their lives," President Obama said in a statement.

Those affected "will have the support of the American people," he said.

The explosion shook homes as far as 50 miles away. It measured as a 2.1-magnitude seismic event, according to the United States Geological Survey.

"It's overwhelming to us," Smith told CNN affiliate KCEN, with blood spattered all over his face from injuries he suffered. The town has only three ambulances, he said.

Between 50 and 60 homes in a five-block area suffered damage, officials said.

Blast struck first responders

The incident began with a fire. Some firefighters came to the scene to quell the blaze, and were there minutes later when the explosion happened at the West Fertilizer Co. at about 7:50 p.m. (8:50 p.m. ET).

"It was like a nuclear bomb went off," he said of the blast. "Big old mushroom cloud."

"(It's) massive -- just like Iraq. Just like the Murrah (Federal) Building in Oklahoma City," said D.L. Wilson of the Texas public safety department.

The blast stripped a nearby apartment complex, with 50 units, of its walls and windows. "It was just a skeleton standing up," Wilson said.

The blast sent a massive fireball into the sky. Flames leaped over the roof of a structure and a large plume of smoke rose high into the air.

"The windows came in on me, the roof came in on me, the ceiling came," said George Smith, the city EMS director.

"It, like, picked you up," a woman told CNN affiliate WFAA. "It just took your breath away. And then it dropped you and it exploded everything around you... It was like a suction and then it just blew it all out. You could feel everything. You could feel it on your skin, your hair was being blown. It was crazy."

She managed to cover one of her children, she said, and "grabbed my little one and dove through a door. It was chaos. All my windows blew out, my doors off the hinges. All I had were my keys in my hand and I just threw the dog, everybody in the car and we took off."

Brad Smith lives 50 miles away and felt his house shake.

"We didn't know exactly what it was," he said. "The forecast said a line of thunderstorms was going to come though. My wife and I looked up and wondered, 'Did it get here six hours early?'"

Five hours after the blast, carloads of the wounded continued to stream into hospitals.

While some of the injuries are minor, others were "quite serious," said Glenn Robinson, chief of Hillcrest Hospital in Waco.

Hillcrest reported five patients in intensive care -- two in critical condition, three in serious condition. At least 28 patients will be admitted, said hospital chief Glenn Robinson.

Shelter needed

The storm system working its way through the area -- which includings ilghtning and hail -- could cause problems not only for firefighters, but also for those left homeless.

Overnight lows will be just above freezing, said CNN Meteorologist Jennifer Delgado.

And the danger may not be over.

Even though officials have turned off all the gas at the plant, they worry another tank at the facility might explode.

"What we are hearing is that there is one fertilizer tank that is still intact at the plant, and there are evacuations in place to make sure everyone gets away from the area safely in case of another explosion," said Ben Stratmann, a spokesman for Texas State Sen. Brian Birdwell.

West is about 75 miles south of Dallas and 120 miles north of Austin. The town's chamber of commerce touts it as "the Czech point of central Texas."

Czech immigrants arrived in the town in the 1880s, and the community still maintains strong ties to their central European roots, with businesses named "Little Czech Bakery" and "The Czech Inn."

The scene

Early Thursday morning, state troopers in gas masks manned roadblocks, waving away cars coming off the highway.

The Federal Aviation Administration instituted a flight restriction over the town.

Authorities closed schools for the rest of the week, and urged everyone to stay away from school property.

So many firefighters and medics descended on the town to help its all-volunteer force that the public safety department pleaded that no more assistance was needed.

"The firefighters and EMS people are coming from hundreds of miles away to help us," Wilson said. "Right now, we are overflowing with help. "

Worst-case scenario

In 2006, West Fertilizer had a complaint filed against it for a lingering smell of ammonia, the Texas Commission on Environmental Quality website shows.

Separately, the plant had informed the Environmental Protection Agency that it presented no risk of fire or explosion, according to The Dallas Morning News. It did so in an emergency planning report required of facilities that use toxic or hazardous chemicals.

The plant's report to the EPA said even a worst-case scenario wouldn't be that dire: there would be a 10-minute release of ammonia gas that wouldn't kill or injure anyone, the newspaper reported.

But what happened Wednesday night was much worse.

Tommy Alford, who works in a convenience store about three miles from the plant, said several volunteer firefighters were at the store when they spotted smoke.

Alford said the firefighters headed toward the scene and then between five and 10 minutes later, he heard a huge explosion.

"It was massive; it was intense," Alford said.

CNN's Pam Benson, Chandler Friedman, Carma Hassan, Ed Payne, Greg Botelho, Amanda Watts, Jake Carpenter, Tina Burnside, Dave Alsup, Tanika Gray, Darrell Calhoun, Ryan Rios, Alta Spells, Travis Sattiewhite and Christabelle Fombu contributed to this report.