TEXAS CITY, TX --Residents of a southeast Texas city were told they could go outside again Tuesday, after officials determined that overnight power outages did not cause three Houston-area refineries and a chemical plant to release unhealthy amounts of harmful gases. The "shelter-in-place" order for Texas City was lifted around noon, about 12 hours after the first alarms sounded warning people to stay inside, said Homeland Security coordinator Bruce Clawson. Residents had been advised twice since Monday night to stay inside after power failures shut down a Dow Chemical plant and all or parts of three refineries, including a BP unit where a 2005 explosion killed 15 people. Texas City schools canceled classes. The order was lifted because air quality monitors showed no harmful emissions, Clawson said. "There was constant monitoring going on at all times and it did not reveal anything, although there's a strong smell of hydrocarbons in the air," he said. Emergency officials were working to determine what caused the outages, which a spokeswoman for Texas New Mexico Power company said involved a "customer-owned equipment problem." Utility spokeswoman Cathy Garber said the company provides power to the affected plants but was not the source of the outages. Garber said four "events" affected transmission lines Monday night, but said she did not have details. Texas City's emergency sirens sounded and residents were advised to stay indoors after the BP refinery and Dow plant lost power about 11 p.m. Monday, Clawson said. Valero and Marathon Oil refineries lost power nearly six hours later. The refineries activated their flaring systems after the outages, sending tall flames shooting from stacks into the sky. Flaring allows the plants to burn off excess gas in order to avoid unhealthy air or explosions. "I knew when them flares were going, something messed up is going on out there," said retired carpenter Dwight Crawford, 66, who lives about a mile from the BP plant and said he had grown accustomed to such disruptions. "It's just a hazard of living close to this plant," Crawford said. "I'm going to try to stay inside, but I do have to go about my daily routine. I can't let the plant shut me down." A fire broke out at the BP refinery shortly after the power went out, but was quickly extinguished, spokesman Michael Marr said. He said there were no injuries at the refinery, the site of the deadly March 2005 explosion that also left 170 people injured. Marathon experienced a brief power dip Monday night and was working to restore the affected units, company spokesman Shane Pochard said. Most of the Valero refinery's units were safely shut down early Tuesday, spokesman Fred Newhouse said. Workers could be seen reporting for their shifts at the BP plant around sunrise Tuesday, while flaring continued as part of the shutdown process. "That's a normal part of bringing down some of these operating units," said BP spokesman Tom Mueller. "In an emergency shutdown like this one, you would send a lot of materials to the flares."