(AP) — Officials were trying to determine Wednesday if a 4.5 magnitude earthquake triggered an explosion at a fuel storage facility in the San Francisco Bay Area that started a fire and kept thousands of people in their homes for hours because of potentially unhealthy air.

The earthquake struck about 15 miles (24 kilometers) southeast of the NuStar Energy fuel storage facility in Crockett. It occurred 15 hours before the fire Tuesday that consumed thousands of gallons of ethanol, a gasoline additive.

Video footage of the fire showed an explosion so strong that it blew the lid from one tank high into the air. The fire badly damaged or destroyed two tanks containing about 250,000 gallons of ethanol.

Contra Costa County Fire Protection District spokesman Steve Hill said it was too early to say what caused the explosion, and a team of government and industry firefighters were investigating. One firefighter had minor injuries.

Aftershocks in the area were still being felt Wednesday, including one with a 3.4 magnitude.

Firefighters have put a foam blanket on top of the remnants of the tanks that were destroyed to prevent remaining ethanol from reigniting the fire.

They also checked a tank that had been exposed to fire for possible damage and it was found to be safe, Hill said.

The seven-hour blaze erupted in towering, stubborn flames about 30 miles (50 kilometers) northeast of downtown San Francisco. NuStar says on its website it has 24 tanks capable of holding more than 3 million barrels of ethanol, gasoline, diesel and aviation fuels.

Emergency sirens blared and a column of thick black smoke that could be seen for miles prompted health officials to order people in Crockett, Rodeo and part of Hercules to stay inside with fans and air conditioners off. They were also told to seal their windows and doors with tape or wet towels amid concerns that hazardous particulates might be spewing from the fire.

County health officials later lifted the shelter in place order affecting about 12,000 people. At least four schools were closed Wednesday as a precaution, even though health officials do not believe there is a public health threat.

“We have been sampling air in the community immediately surrounding the facility checking for particulate matter and other stuff and the air doesn’t appear to be unusually high with anything that we would be concerned about,” county health department spokesman Karl Fischer said.

About 200 firefighters fought the flames with foam and water, trying to prevent it from spreading to other tanks containing jet fuel and ethanol. Flames kept reigniting in the spilled fuel.

The earthquake caused malfunctions at two nearby oil refineries operated by Shell and Marathon, Randy Sawyer, Contra Costa County’s chief environmental health and hazardous materials officer, told KQED News.