Two workers who were trapped in an Oklahoma dam following an explosion Wednesday night have died, officials confirmed Friday. 

The explosion occurred just before 6 p.m. at Kerr Dam, southeast of Tulsa, as contractors were doing routine core drilling about 80 feet down from the top of the dam, the Grand River Dam Authority – which oversees the structure – said. 

"At approximately 4:30 a.m. this morning, Friday, May 14th, recovery crews were able to recover the bodies of two men who had been trapped inside Kerr Dam," John Wiscaver, the executive vice president of GRDA, said in a statement obtained by FOX23. "Both men were contractors. No GRDA employees were involved in the incident." 

The Kerr Dam in Oklahoma, where the explosion occurred. 

The Kerr Dam in Oklahoma, where the explosion occurred.  (Google Maps)

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Wiscaver added that a full investigation into the incident is underway. 

There were three contractors drilling at the time of the blast, and one was able to make it out, Justin Alberty, a spokesman for the Grand River Dam Authority, told KJRH-TV

That worker initially refused treatment, but was later taken to a hospital in Pryor, Oklahoma, according to the station. 

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Alberty noted the explosion didn't compromise the structure of Kerr Dam. The cause of the blast has yet to be determined.  

The core-sample drilling was part of a normal safety maintenance program, according to the GRDA.