Officials say he was underwater four to seven minutes, resuscitated after 50 minutes of CPR but later died from brain swelling.
His best friend says Avi was one of a kind.
"Avi was uniquely wonderful. The currency he traded in was friends. Everyone felt that Avi was their best friend and hew as uniquely able to make you feel happy and feel good about yourself at all times," says Dan Gemp, best fried of Avishek Sengupta.
Friends say Avi was in his best shape. He trained hard and was excited for the Tough Mudder. It's a 10-plus mile run over rugged terrain with around 20 obstacles.
Tough Mudder released a statement saying they are deeply sorry for Avishek's loss. They say safety is their top priority: in over 50 Tough Mudders, and a total of 750,000 participants, he is the only death.
"They have a lot of crew around. They're constantly looking and talking to you, asking you if you're okay," says Nazim Jafferali, participant in the Mid-Atlantic Tough Mudder.
Police report at least 10 injuries from Saturday's event, including those flown from the scene to the hospital. One participant says she was also injured on a water obstacle called "Pirate's Booty."
"I hit the ground, and apparently when I hit the ground, I hit a guy. I went under the water and when I came back up, I was facing down. Somebody flipped me over," says Jennifer Lanberg Andersen, participant in the Mid-Atlantic Tough Mudder.
Thousands of so-called Mudders crossed the finish line this weekend, and say it lives up to its name.
"You have to have cardio and upper body strength to get through a lot of the obstacles, it's all mind over matter," says Jacob Brady, participant in the Mid-Atlantic Tough Mudder.
Mudders came together as a community on social media to recognize the dangers and excitement, and to mourn the loss of Avishek Sengupta.
The Berkeley County Sheriff's Office is conducting the investigation and believes Avishek's death was a tragic accident.