Cause Unknown

"Nothing at this point indicates we have had criminal activity, but we are not ruling that out," said Swanton. 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.