(Reuters) – Four people were injured when a fire erupted on Thursday at an ExxonMobil Corp (XOM.N) complex in Baytown, Texas, one of the largest refining and petrochemical facilities in the United States.

There were no fatalities and two of four contract workers hospitalised with injuries were released after treatment. The two still in hospital are in stable condition, the company said.

The cause of the blaze, which was reported about 1:00 hours CT (0700 GMT) and extinguished seven hours later, has not been determined, Exxon said.

The fire occurred in a hydrotreater unit at the refinery that had been shut on Wednesday due to a bypass line leak. The injured were contractors who had been repairing the leak, people familiar with plant operations told Reuters.

Hydrotreaters remove sulfur and other contaminants in producing diesel, gasoline and jet fuel. An Exxon official told a news conference that the fire had affected a unit that produces gasoline.

Harris County Sheriff, Ed Gonzalez, said on Twitter initial reports had indicated that there had been some type of explosion at the plant. Social media users said on Twitter that a blast shook buildings in the area.

The Baytown plant houses a chemical plant, an olefins plant and the country’s fourth-biggest oil refinery, with capacity to process 560,500 barrels per day of crude. It is unclear whether any units will be shut for an extended length of time.

Exxon is adjusting production rates at other Baytown processing units to focus on stabilising the affected units, spokeswoman Julie King said.
The facility spans about 3,400 acres along the Houston Ship Channel, about 25 miles (40 km) east of Houston, and employs about 7,000 people. The company has set up a hotline for local residents to seek claims for damages.