(CNN) -- Some 35 people -- including 10 first responders -- died in an massive explosion Wednesday night at a fertilizer plant in West, Texas, Mayor Tommy Muska said, according to USA Today.
The number included five volunteer fire fighters, four emergency responders and an off-duty fire fighter from Dallas who lived in West, the mayor told the newspaper.
Dallas Mayor Mike Rawlings identified the off-duty fire fighter as Kenny Harris, a captain in his city's fire department. Harris "rushed to the scene as a helper," Rawlings said on Twitter.
Officials at news conferences in West were unwilling to give any numbers on victims. They have only confirmed there have been casualties.
Earlier, Waco Police Sgt. William Patrick Swanton estimated there were five to 15 people who died.
George Smith, West's director of emergency services, has said the death toll could spike to 60 or 70.
More than 160 people were injured.
The area around the site of the massive explosion at a fertilizer plant that flattened much of the small Texas town remains "very volatile" due to the presence of ammonium nitrate, McLennan County Chief Deputy Sheriff Matt Cawthon said Thursday.
Authorities searched through mounds of rubble in hopes of finding survivors of the blast that left shattered homes and wreckage in a wide swath of the town, which has only 2,800 people.
"It's overwhelming to us," said Smith. As he spoke to CNN affiliate KCEN, blood was spattered on his face from injuries he suffered.
"It was like a nuclear bomb went off," said Mayor Tommy Muska.
Muska told CNN on Thursday afternoon that emergency officials were still combing through a nearby nursing home, apartment complex and the plant looking for survivors.
"We still are holding out some hope, but right now we're just trying to get a hand around it and see," he said.
At the same time he said he realized the casualty count might rise as the number of missing falls.
"We've got the best of the best looking, and that's what we want to do," he said. "I want to count up all my citizens and all my firefighters."
On Wednesday evening, a fire at the plant suddenly exploded with a huge, deafening bang, throwing people to the ground blocks away.
About half the town was evacuated, including the nursing home with 133 residents.
Three schools also are near the plant. Classes weren't in session when the explosion happened Wednesday night.
The explosion measured as a 2.1-magnitude seismic event, according to the United States Geological Survey. Even 50 miles away, homes shook.
Texas Gov. Rick Perry declared McLennan County a disaster area.
'Roof came in on me'
The explosion tore through the roof of West Fertilizer Co., charring much of the structure and sending massive flames into the air, followed by a plume of smoke bigger than the plant itself. A deafening boom echoed for miles.
It was "massive -- just like Iraq, just like the Murrah (Federal) Building in Oklahoma City," said D.L. Wilson of the Texas public safety department, referring to a bombing that took place 18 years ago Friday.
The blast stripped the apartment complex, with 50 units, of its walls and windows. "It was just a skeleton standing up," Wilson said.
"The windows came in on me, the roof came in on me, the ceiling came," Smith said.
The town, he said, has only three ambulances.
Between 50 and 60 homes in a five-block area sustained damage, officials said.
"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."
Video taken by college student Eric Perez -- from what appeared to be a safe distance away from the fire, past a large field -- shows the moment of the blast. In a split second, the fire over the plant bursts into into widespread destruction, knocking Perez over. Bright flames fill the sky.
Perez and his friends were playing basketball when they noticed the flames at the plant. "Nothing ever happens in our small town, so I recorded," he said. "Then the explosion went off. We were thrown into the bed of my truck."
One of his friends suffered a flash burn to his face and broken ribs. Another was burned on his arm and hit with flying shrapnel. A third had a cut to his face.
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 through. My wife and I looked up and wondered, 'Did it get here six hours early?' "
As of Thursday afternoon, authorities had not determined what led to the deadly explosion. Cawthon said his sheriff's office; the federal Bureau of Alcohol, Tobacco, Firearms and Explosives; and the state fire marshal's office are working "to determine the exact cause of the situation."
Swanton said earlier there were no indications of criminal activity but that wasn't being ruled out yet.
Texas Attorney General Greg Abbott told reporters "it's way premature" to determine whether any criminal charges could be sought in relation to the deadly explosion.
A U.S. intelligence official told CNN there is no indication so far that the blast is related to terrorism.
While state authorities are leading the investigation, the federal government is assisting.
The ATF 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 said Thursday.
The White House said it is monitoring the situation through the Federal Emergency Management Agency, which is in touch with various agencies involved in the investigation.
With help from heavy rains early Thursday, firefighters managed to quell most of the flames in the area, authorities said.
The rain and heavy winds also helped dissipate chemicals that may have been released.
Swanton emphasized that there was no cause for alarm about the air. There was no "chemical escape" that is "out of control," he said.
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.
But doctors reported that the injuries they were treating, in general, came from the blast, not chemical exposure. Many people had cuts and puncture wounds. Some were in critical condition.
Small, smoldering fires remained Thursday, but not enough to halt rescue efforts, police said.
There was one early report of possible looting, but no one was arrested, Swanton said.
The Federal Aviation Administration instituted a flight restriction over the town.
Local rail freight service was also being closed to help secure the area, the U.S. Department of Homeland Security said.
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 Texas public safety department said that no more assistance was needed.
"The firefighters and EMS people are coming from hundreds of miles away to help us," said Wilson of the department. "Right now, we are overflowing with help. "
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 its Central European roots, with businesses named "Little Czech Bakery" and "The Czech Inn."
In 2006, the Environmental Protection Agency fined the company that ran the fertilizer plant $2,300 and told the owners to correct problems, an EPA spokesman told CNN.
David Gray said the company certified that it had fixed the deficiencies, which included a failure to file a risk management program plan on time.
Also 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 2011 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.
What happened Wednesday night was much worse.