GINA SUNSERI, MICHAEL S. JAMES & CHRISTINA NG // ABC NEWS -

The mayor of the small Texas city devastated by an explosion at a fertilizer plant said that 35 to 40 people are unaccounted for.

Volunteer firefighters are among the missing and authorities initially said they feared that five to 15 people could be dead. Officials later said that they would not give out any casualty numbers.

Mayor Tommy Muska told ABC News he believed that 35 to 40 people have not been accounted for.

Search and rescue teams in the town of West, Texas, are looking for survivors and missing people amid buildings where walls and roofs have been torn away and other buildings have been flattened by an explosion at a fertilizer plant.

"We have confirmed fatalities at this time. We don't have the exact amount," Texas Department of Public Safety Sgt. Jason Reyes said at a news conference this afternoon.

One of the victims was Dallas Fire-Rescue Captain Kenny Harris who was initially reported missing, according to a news release from the city of Dallas.

Harris was off-duty at the time of the blast and was not a volunteer firefighter, but responded as a helper. He was not believed to have been involved in any firefighting activity, according to the statement.

"Captain Harris' response is typical of all our first responders; night and day, no matter where they are, no matter if they are on or off duty they respond with the greatest acts of bravery," City Manager Mary Suhm said in a statement. "The City of Dallas and the citizens of Dallas have lost a real role model."

The explosion devastated the area that officials described as a highly-populated neighborhood.

"It ranges from broken windows to complete devastation," Waco Police Department Sgt. William Swanton said at a news conference today. "There are homes that are no longer homes."

At some buildings, "walls were ripped off, roofs were peeled back," the sergeant said.

The fire and explosion Wednesday night in a small town north of Waco prompted widespread evacuations and sent more than 160 injured people to hospitals.

The blast at the West Fertilizer Plant in West, Texas, occurred just before 8 p.m., but officials still were struggling to tally the dead and injured early this morning and searching door-to-door amid the rubble for survivors, police said.

Earlier today, authorities expressed concerns about looting, but now say they believe what was initially reported to them was an isolated incident.

"I have confirmed at least there was an incident last night when they thought they may have had a looter," Swanton said, adding that the incident occurred "very, very early in the scenario."

He said there was no arrest and the problem is "not rampant," but people are still being kept out of the main disaster area.

Swanton said the five to 15 deaths is a "rough number" and they are unverified.

"I don't have a number of how many they have rescued or how many potential bodies they have found," he said.

"There are still firefighters missing," Swanton said.

He said an estimated three or four who are missing are volunteer firefighters, "meaning that they probably have a very large contingent of people that are willing to risk their lives for the neighbors and community."

They are the first responders who were battling the fire when the explosion occurred, he said.

A firefighter and law enforcement officer who was previously mentioned as missing has been found, Swanton said. He is in a hospital with "pretty serious injuries," he added.

Swanton said authorities are still in search-and-rescue mode and are not yet in recovery mode.

"The town is secure. There are plenty of law enforcement officials that are stationed around the town," Swanton said. "There is no fire out of control. There is no chemical escape from the fertilizer plant that is out of control."

The Red Cross, mental health agencies and grief counselors are on hand to help the community, in addition to the neighbors who are already assisting each other.

"That is a very tight-knit, very family packed, family-oriented community," Swanton said of the town of about 2,800 people. "They are leaning on each others' shoulders."

West EMS Director Dr. George Smith, himself injured and bloody, said he had not personally seen bodies to confirm deaths, but believed the blast killed at least two emergency responders to a fire at the plant before the initial explosion and a person at a nearby apartment complex that suffered serious damage.

In addition, some responders to the fire were believed unaccounted for after the blast, according to Smith, Swanton and West Mayor Tommy Muska.

As they gained access to the explosion site, officials said they were treating it as a crime scene.

"We are not indicating that it is a crime, but we don't know," Swanton said. "What that means to us is that until we know that it is an industrial accident, we will work it as a crime scene. ATF [the federal Bureau of Alcohol, Tobacco, Firearms and Explosives] is conducting the main investigation."

President Obama, in a statement, extended his condolences to the people of West and thanked first responders.

"A tight-knit community has been shaken, and good, hard-working people have lost their lives," he wrote.

"My administration, through FEMA and other agencies, is in close contact with our state and local partners on the ground to make sure there are no unmet needs as search and rescue and response operations continue."

Texas Gov. Rick Perry said he has requested an emergency declaration from the president for West.

"Last night was truly a nightmare scenario for that community," Perry said at a news conference today. "Anyone who grew up...in a small town like West, they know that this tragedy has most likely hit every family, has touched practically everyone in that town."

The disaster even drew condolences from Pope Francis, who tweeted, "Please join me in praying for the victims of the explosion in Texas and their families."

Earlier concerns about the possibility of dangerous ammonia fumes and shifting winds subsided by morning as fires died down, Swanton said before 6 a.m. ET.

"Air quality, at this point, is not an issue," Swanton said. "It is not a concern."

Nevertheless, numerous other concerns remained.

Witnesses reported heavy fire or concussive damage to a middle school, homes and an apartment complex near the plant, as well as to a nursing home, where more than 130 residents were evacuated, Mayor Muska said.

Buildings in a radius of about five blocks around the plant -- including at least 60 more homes -- were heavily damaged by the blast, officials said.

"It was almost tornadic in effect," Swanton said. "It looked like to me one home would be fine and next to it there would be extreme devastation."

State Trooper D.L. Wilson of the Texas Department of Public Safety described the initial fertilizer plant blast as "massive -- just like Iraq, just like the Murray Building in Oklahoma City. The same kind of hydrous [ammonia] exploded, so you can imagine what kind of damage we're looking at."

The blast even registered as a 2.1-magnitude seismic event, according to the U.S. Geological Survey.

It was felt 20 to 30 miles away, witnesses said, and near the plant it burned buildings, knocked down people, blew out windows and, according to Wilson, left the damaged apartment complex looking like "just a skeleton standing up."

"It's total chaos," West City Councilwoman Cheryl Marak said soon after the blast, according to ABC News Radio. "There's ambulances and fire trucks and police cars from everywhere."

Marak told ABC News that the explosion killed her pet dog and destroyed her house about 2 1/2 blocks from the plant, as well as houses around it.

"With the explosions, the whole street lifted up," she told ABC News. "It was like a massive bomb went off. It demolished both my houses, my mother's and mine."

"I think everything around us is pretty much just gone," she added, according to ABC News Radio.

Keith Williams, a local resident, said his house also was destroyed.

"All the ceilings are out," Williams said, according to ABC News Radio. "The windows are out. The brick's knocked off the house. My big garage out back is half blowed in."

He also saw "people with all their houses tore up across the street from me, on each side of me."

By 5:45 a.m. ET, hospitals near the blast site reported treating 180 people. At least 16 patients at the hospitals were in critical condition and three in serious condition.

Hillcrest Baptist Medical Center in Waco, Texas, saw more than 100 of the wounded, officials there said. Patients from the blast also were confirmed early Thursday at Providence Healthcare Network in Waco, Parkland Hospital in Dallas, and Scott & White Memorial in Temple, Texas.

The fertilizer plant exploded around 7:50 p.m. local time Wednesday, according to the Texas Department of Public Safety.

Emergency response audio told the story of the chaos among firefighters and others at the scene.

"We need every ambulance we can get this way," one snippet said. "A bomb just went off. It's pretty bad."

"Firefighters down," another said. "There has been an explosion."

"The rest home has been seriously damaged. We have many people down. Please respond."

There were subsequent explosions around 10 p.m., ABC News affiliate WFAA reported. The cause of the explosions was unconfirmed, but a dispatcher was heard warning crews to move away from chemicals in unexploded tanks.

The fertilizer plant was fined $2,300 by the Environmental Protection Agency in 2006 for failing to have a risk management plan that met federal standards, the EPA told ABCNews.com in an email.

The plant was not penalized by the EPA again after that incident and has not had any major accidents in the last five years.

Another EPA report showed that West Fertilizer Co. reported the "worst possible scenario…would be a 10-minute release of ammonia gas that would kill or injure no one," according to the Dallas Morning News.

The town of West has a population of about 2,800.

ABC News' Leezel Tanglao, Clayton Sandell, Elizabeth Stuart and Gina Sunseri contributed to this report.