A massive explosion 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.

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.

Nearby homes sustained significant damage. Some were knocked off their foundations.

Manny Jawa, a volunteer with the Houston office of the American Red Cross, said his agency had helped about 100 people who came to an evacuation shelter that had been set up around 6 a.m. Friday at a church near the blast site. The shelter offered residents food and water and information about help they could receive.

Officials said that they planned to open an overnight shelter at a different church for residents who had been displaced from their homes and had nowhere else to go.

Danny Wilson, 63, who lives less than a mile from the site, said he was sleeping when his wife woke him up.

“She heard a big noise and the (grandkids) were running out of their rooms,” Wilson said. “She said it was some kind of explosion or somebody was trying to get in.”

Wilson said he first checked inside his home to make sure nobody had broken in and then he went outside and talked to neighbors to find out what was going on and to check for any damage.

“I didn’t notice any broken glass and I looked at the back window and it was shattered big time,” Wilson said.

He said the blast also broke glass on part of his front windows.

“Everybody seems to be OK now. That’s the main thing,” Wilson said.

Miguel Ramirez, 65, tried to get out of his bedroom to see what had happened, but his bedroom door would not open.

Ramirez said he had his son hand him a small screwdriver through an opening underneath the door and he used that to remove the pins from the door’s hinges so he could get out.

Once he got out, Ramirez said he found that a large portion of the ceiling in his living room had collapsed onto the floor and sofa. Chunks of insulation were on the carpeted living room and on the couch. The wooden beams on the ceiling were exposed.

The explosion also shattered the sliding door in his kitchen that leads to his backyard. Bits of glass were strewn all over the kitchen floor and a dining room table that still had plates from when the family ate Thursday night. The blast also knocked a light fixture from the ceiling in the kitchen.

“The good thing is nobody got hurt,” said Ramirez, who lives about 500 feet (150 meters) away from the company where the explosion occurred. He lives in the home with his wife, son and daughter.

The blast shook other buildings, with reports on Twitter of a boom felt across the city. Pena said there were no reports of hazardous air quality, based on monitoring done by a hazardous materials team.

The family-owned business manufactures valves and provides thermal-spray coatings for equipment used in the chemical, mining, petroleum and aerospace industries, among others, according to its website. In a statement Friday night, Western Grinding said it is “working diligently to address the situation and cooperating with the federal, state and local authorities investigating the accident.”

The statement also said: “Our hearts go out to the families and businesses impacted by this incident and to our community. At this time our immediate concern is the safety and wellbeing of everyone in the area and our employees.”

About 90,000 people live within 3 miles (4.8 kilometers) of the company, according to the Environmental Protection Agency. Houston police tweeted that officers were blocking off streets, but no evacuation was ordered. Harris County Sheriff Ed Gonzalez said first responders checked on residents of nearby homes.

Turner said on Twitter said there was no hazard in the air “from all indications,” but that authorities continued to monitor.

This part of Texas is home to the highest concentration of oil refineries in the nation and has experienced a series of explosions in recent years. Last July, an explosion at an ExxonMobil refinery in Baytown, east of Houston, left more than dozen people with minor injuries and put nearby residents under a shelter-in-place advisory for three hours.

In December, two blasts in the coastal city of Port Neches shattered windows and ripped the doors from nearby homes.

Two homeowners are already suing Watson Grinding, accusing the company of negligence and demanding more than $1 million in damages. According to one lawsuit filed by homeowner Sophia Navarro, the blast blew out the windows of her home, with glass strewn inside and outside, and fractured the home’s walls and foundation.

“This is not the first time Houston has seen massive explosions, nor is it the first time that widespread areas of Houston and surrounding areas have suffered from the effects of such a massive explosion,” the lawsuit says.

(AP)