A fisherman swam into a river and rescued a friend who had stumbled into deep water while untangling a fishing line, but then he died while swimming back to the riverbank, police said.

Domingo Acosto, of the Bronx, went out fishing in the Hudson River yesterday morning with his friend Wenceslao Guzman and Mr. Guzman’s 10-year-old son in an area near the Tappan Zee Bridge called Sandy Beach.

Mr. Guzman, whocouldn’tswim, told police his fishing line got snagged on something in the river and he waded in to unhook it. But the water was deeper than he thought, and he went under and started swallowing water and panicking, police said.

Acosto swam about 15 feet to Guzman, who was thrashing around, and pushed him toward the river’s edge until he reached shallow water and was able to stand up. Mr. Guzman told police that as he got out of the water he saw Acosto swimming to the riverbank.

But when he looked again, he said, Acosto was lying face down in the water and wasn’t moving. His son called police, and rescue boats responded. Acosto, 60, was taken to an ambulance waiting at a nearby marina. He was pronounced dead on arrival at a hospital.

The Westchester County medical examiner scheduled an autopsy for Friday to try to determine what killed Acosto.

“He’s swims out and saves his friend, and he ends up dead,” Tarrytown police Detective Sgt. Eugene Buonanno said. “Something went drastically wrong.”