On Saturday 27 April at about 6pm a fire explosion at the refinery was reported, following which residents in the town of Melvindale, Michigan, were asked to evacuate the area.
The fire was dosed off by 8pm and no injuries were reported.
Marathon Petroleum spokesman Shane Pochard was quoted by Reuters as saying that the investigation of the incident pointed out that the fire was result of a blaze in a tank containing wastewater from the refining process.
"We’re working with Melvindale police to return residents to their homes," Pochard added.
"That has not yet been completed."
The tank contains what is called sour water, which includes hydrogen sulfide and ammonia from crude oil refining, which are removed from the water before it can be reused or sent back to an outside wastewater system.
Marathon Petroleum is monitoring the air quality in the area following the fire.
The refinery has a processing capacity of 106,000 barrels of crude oil per day.
