Police were searching Saturday for a retired medical school professor and AIDS researcher from North Carolina who went missing during a marathon swimming race down New York's Hudson River.

Authorities say Dr. Charles van der Horst, 67, was reported missing just after 3 p.m. Friday near the George Washington Bridge. Police say a man who was watching from the river bank saw van der Horst disappear and called 911.

The New York City Police Department resumed searching for van der Horst on Saturday after suspending the search late Friday.

Van der Horst was taking part in a multi-stage 120-mile (193-kilometer) race down the Hudson called the 8 Bridges Race. Organizers canceled Saturday's seventh and final stage of the race.

New York Open Water, the organization that runs the race, said in a statement, "Our thoughts are first and foremost with the Van Der Horst family and we ask that all respect their wish for privacy as they mourn this tragic loss."

The group said 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."

Van der Horst 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."