A swimmer disappeared in the Hudson River during a marathon swim, police said.
Charles van der Horst, 67, off Chapel Hill, North Carolina, was participating in the 8 Bridges Hudson River Swim around 3 p.m. Friday when a safety spotter saw him submerge near the George Washington Bridge, according to police.
Emergency crews started searching for the man as soon as he went missing, but suspended their search late Friday night, according to the Coast Guard.
The search resumed Saturday.
Van der Horst was taking part in a multi-stage 120-mile race down the Hudson. Organizers canceled Saturday's seventh and final stage of the race.
New York Open Water, the organization running the event, says safety protocols were in place and police were escorting the swimmers.
Van der Horst retired from the University of North Carolina, Chapel Hill medical school, where his career encompassed clinical medicine, teaching and research. He has served as a consultant helping to implement AIDS treatment and prevention programs in South Africa and as a volunteer physician at a free clinic in Raleigh.
Van der Horst was among more than 900 people arrested at a 2013 demonstration against a North Carolina law that prohibited transgender people from using the public restroom of their choice, the News and Observer reported.
A friend of van der Horst, author Tim Tyson, remembered the arrests Saturday in a public post on Facebook, saying they "landed us in the same police van and some other places where our souls proudly touched."
Tyson said many people loved van der Horst for "his sweet spirit, his pride in and love for his family, his deep and energetic love of humanity, his well-informed devotion to the common good, his kindness to everyone around him, his zesty embrace of life, his hilarious sense of humor and the absurd, and his unassuming and unflagging willingness to make sacrifices for his most deeply help moral values."
He wrote about participating in previous swimming races down the Hudson in a column that appeared in the newspaper last year.
"Racing 15 miles in the Hudson River beneath the cliffs of West Point, dwarfed by an oil tanker with its propellers moving whump, whump, whump like some whale in heat, brought perspective as to the vastness of nature," van der Horst wrote.
He wrote that when the waves tossed him "like a piece of flotsam" he embraced "the calm knowledge that I could ride them out despite my primal fears of the immense crushing power."