West, Texas Mayor Tommy Muska said that 35 to 40 people are dead from the massive explosion that leveled a quarter of the town.

Muska said he arrived at the estimated death toll based on the number of people who have not been identified by local officials. Among the missing and believed dead are six firefighters and four emergency medical technicians.

Rescuers continue to comb through the remains of West, Texas hoping to find survivors still buried beneath the rubble of houses and businesses. Rescue officials refused to call the day’s work anything but a search and rescue operation.

The explosion, so intense it registered as a 2.1 on the Richter earthquake scale, levels dozens of homes, a school and a nursing home. Residents of Waco, Texas, 20 miles north of the blast site, reported hearing the explosion Wednesday night.

A spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives said the site was still too hot for investigators to work the site. No timetable has been set to begin the investigation as to the cause of the fire that led to the blast.

The Dallas Fire-Rescue Department released a statement that one of their off-duty firefighters, Capt. Kenny Harris, was killed in the blast. Harris, a resident of West, decided to lend a hand to the volunteer fire department who fought the initial fire.

Harris leaves behind a wife and three grown sons.