BAYTOWN, Texas — Crews have extinguished a large fire at a Houston-area oil refinery that broke out early Thursday, injuring four people.

The massive fire and a possible explosion occurred at the ExxonMobil Baytown complex in Baytown, Texas. 

Baytown is located about 25 miles east of Houston, within Harris and Chambers counties.

According to officials, the blaze broke out about 1 a.m. The Baytown Fire Department in a news conference said the fire affected a unit that processes materials with gasoline components.

Some nearby residents reported hearing an explosion. 

The four people injured were taken to hospitals and have been stabilized. Everyone else present when the fire broke out has been accounted for.

Three of the injured were taken to hospitals by helicopter while the fourth was taken by ambulance, Harris County Sheriff Ed Gonzalez said.

The cause of the fire is not yet known. ExxonMobil said air quality monitoring has not revealed any issues. No evacuations or shelter-in-place orders have been issued.

The refinery, located along the Houston Ship Channel, has the capacity to process up to 584,000 barrels of crude oil each day. It employs about 7,000 people, according to the company’s website.

The Associated Press contributed to this report.