A Houston-area man drowned Sunday evening while swimming with his grandchildren in Galveston.

The Galveston County Daily News reports police say the swimmer is 66-year-old Aleksandar Momcilovic of Pearland.

They say Momcilovic was on a boogie board with his grandchildren in the water off the seawall. His grandchildren noticed Momcilovic bobbing up and down with his fist on his chest and thought he was playing.

The grandchildren ran out of the water to get their mother when they realized he was unresponsive.

Police say Momcilovic was a good swimmer but had a history of medical issues. The exact cause of the drowning is unclear.