Multiple assets were called out to reports of a man in his 50s going underwater and not resurfacing.
Austin-Travis County EMS says the incident has already switched to recovery mode from rescue mode as the swimmer has not been located.
Crews were initially called out to the 4300 block of Mansfield Dam Rd., according to ATCEMS. Rescuers were also searching Tom Hughes Park for the missing swimmer.
No other information is currently available.