Rescue crews resumed searching Thursday for a man missing in the Potomac River near Sandy Landing, deploying sonar and a submersible drone in waters too dangerous for divers.

One more day remains under the fire department's standard search protocol before operations end.

Montgomery County Fire and Rescue Service Assistant Chief Dan Ogren told Bethesda Magazine on Wednesday that if Thursday's effort fails, crews will likely return Friday morning, weather permitting. After that, the search ends. MCFRS standard protocol calls off operations after three days.

The missing man, about 25 years old, entered the river on the Virginia side around 1:30 p.m. Tuesday. He was last seen near Spotzbergen Rock, just south of Sandy Landing, where Difficult Run meets the Potomac in the Great Falls area. A second swimmer managed to reach the Virginia shoreline and was treated by Fairfax County Fire and Rescue.

Tuesday's initial response brought nine swift-water boats from Montgomery and Fairfax counties, sonar, underwater drones and K9 units. Crews searched nearly four hours before severe weather forced them off the water. Wednesday's search lasted four to five hours with no success, Ogren said.

Visibility in the river is so poor that divers cannot safely enter. Ogren has described the riverbed near Great Falls as uneven, rocky, and full of debris, with depths reaching 65 to 70 feet in places.

"The reason why it's illegal to swim in the river isn't because we're trying to be mean to people, it's because that river is deadly," Ogren told DC News Now on Wednesday. He estimated the river claims between five and 10 lives every year.

This is the third swimmer to go missing in the same stretch of the Potomac this summer. Nazir Bell, a 20-year-old Towson University student and Bethesda-Chevy Chase High School graduate from Germantown, disappeared June 14 and was recovered June 16. A 16-year-old boy disappeared July 2 and was recovered the next morning south of Sandy Landing.

The pattern has prompted calls for stronger enforcement. Nazir Bell's parents, Jamaal and Shante Smith, have urged Montgomery and Fairfax county authorities to increase patrols, Fox 5 reported Wednesday. Montgomery County Assistant Chief Administrative Officer Earl Stoddard said in early July the county was considering increased enforcement in partnership with U.S. Park Police during the hottest months. The National Park Service has added warning signs along trails near the river.

Swimming and wading in the Potomac River near Great Falls is illegal.

The outcome of Thursday's search had not been confirmed as of publication. If the man is not found by Friday, July 31, MCFRS will call off operations under its three-day protocol.