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Three people were killed and six others injured after an explosion on a docked towboat on the Tennessee River in Kentucky on Friday, state police and the U.S. Coast Guard said.

Authorities are investigating what caused the blast on the vessel, which was moored and being repaired at the First Marine facility in Calvert City, according to Coast Guard spokeswoman Lexie Preston.

At least 20 people were doing work on the boat at the time of the explosion, Preston said. A preliminary investigation showed the explosion caused a flash fire inside the boat, Kentucky state police said in a statement.

State police identified the dead as Timothy L. Wright, 52; Jerome A. Smith, 56; and Quentin Stewart, 41. They were pronounced dead at the scene, and autopsies are scheduled to take place Saturday morning, police said.

The wounded were taken to local hospitals, according to Preston.

Kentucky Sen. Rand Paul tweeted condolences: