This photo shows the damage at the site of an explosion in Houston, Texas, US, on Friday. Reuters

A massive explosion on Friday leveled a warehouse in Houston, killing two workers, damaging nearby buildings and homes and rousing frightened residents from their sleep miles away, authorities said.

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The explosion happened about 4:30 a.m. inside a building at Watson Grinding and Manufacturing, which makes valves and provides thermal-spray coatings for equipment in various industries, authorities said. The building was reduced to burning rubble and debris, and some of the surrounding buildings suffered heavy damage to parts of their walls and roofs.

Killed were employees Frank Flores and Gerardo Castorena, Police Chief Art Acevedo said at a Friday evening news conference. The two had arrived at their workplace early and were using its fitness gym when the blast happened, Mayor Sylvester Turner said.

Authorities believed there were no other fatalities but another 20 people were injured, two other warehouse workers and 18 others from neighboring homes and businesses, Turner said. None of the injuries were thought to be life-threatening.

Acevedo had said earlier that a family member of one of those killed was a U.S. Marine currently training at Camp Lejeune, North Carolina, and called on the Marines to let the man return to Houston.

Local and federal officials will be at the scene for the next three to four days working through the ruins to determine what caused the explosion, Acevedo said. Authorities don't believe the explosion was intentional though a criminal investigation is underway, he said.

Houston Fire Chief Samuel Pena said hazardous materials crews have secured the valve on a 2,000-gallon (7,571-liter) tank of propylene that had been leaking. Propylene is a colorless gas used to produce chemicals in plastics, synthetic rubber and gasoline. It is highly flammable and can explode in a fire. People exposed to propylene can become dizzy and light-headed, and the gas can also cause liver damage.

Associated Press