The body of a retired AIDS researcher who went missing while participating in a marathon swim race in the Hudson River was recovered on Tuesday.

Charles Van Der Horst, 67, of Chapel Hill, North Carolina, was last seen while participating in the 8 Bridges Hudson River Swim, which organizers say is the longest marathon swim in the world. He was last seen near the George Washington Bridge on Friday.

On Tuesday, a person kayaking discovered his body in the Hudson River, and alerted the authorities, the New York Police Department confirmed to NBC News. Van Der Horst’s body was discovered unconscious and unresponsive and was brought to the Dyckman St. Boat Marina where he was pronounced dead. A medical examiner is currently determining his cause of death.

Van Der Horst worked at the University of North Carolina before his retirement, where he was an internationally recognized researcher. The university mourned his loss in a tweet, saying he “helped develop groundbreaking treatment protocols for HIV/AIDS & inspired a new generation of scientists to tackle Ebola.”

In a statement from Van Der Horst’s family, they said, “He put all of his passion and zest into everything he did, from his love of his family, friends and community, to his swimming to his work on social justice and in the medical field.”

New York Open Water, which organizes swim race where Van Der Horst died said at least 15 other people were swimming in the marathon at the time of his death.

“We can confirm that all swimmer safety protocols were in place and that the NYPD was escorting the field. We will continue to work closely with the authorities,” New York Open Water said. “Our thoughts are first and foremost with the swimmer’s family.”