LOCUST GROVE, Okla. (AP) — Two men were killed and a third person was injured in an explosion in a northeastern Oklahoma dam where they were doing maintenance work, officials said.

Grand River Dam Authority spokesperson Justin Alberty said Kerr Dam, about 45 miles (72 kilometers) east of Tulsa, was not damaged in the explosion about 6 p.m. Thursday,

The men were contractors, GRDA Vice President John Wiscaver said, and the cause of the blast is under investigation. He said their bodies were recovered early Friday. Names weren’t immediately released.

The explosion occurred as the workers were doing core-sample drilling as part of a normal safety maintenance program, Alberty said. The injured worker was taken to a hospital in undisclosed condition.

The GRDA operates dams in northeastern Oklahoma to generate electricity for customers that include cities in Kansas and Oklahoma.