A fire over the summer at a New York City sewage treatment plant may have been caused by a loose nut.

The New York City Department of Environmental Protection released its findings in report on Tuesday.

The report says the cause of the four-alarm fire at the North River Wastewater Treatment Plant in Harlem cannot be determined with certainty, but says a nut connected to a fuel injector had been tightened improperly.

The fire sent hundreds of millions of gallons of raw sewage into waterways surrounding the city in July, forcing several beach closings.

DEP Commissioner Carter Strickland said the department has taken steps to improve safety at the plant.

The plant has been in operation since 1986 and treats an average of 120 million gallons of wastewater daily.