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The official autopsy report won’t be released for several months, but the Manassas County medical examiner’s office has determined that the death of 28-year-old Avishek Sengupta was the result of accidental drowning while participating in his first Tough Mudder on Saturday. The question remains as to why a seemingly healthy young man—an avid rock climber who trained kids at a local gym—ran into trouble on an obstacle that didn’t seem particularly demanding.

Here’s what we know: Saturday morning, Sengupta and five of his friends drove from Baltimore, Maryland, to Gerrardstown, West Virginia. The men arrived at the course late, but it was still cold out when they climbed into the start corral around noon—52 degrees, with bone-chilling 23 mile-per-hour winds.   

“It went as well as you could expect,” said Josh Muskin, one of five teammates who met through working at a small digital marketing firm called WebMechanix. Their team, ranging in age from 23 to 34, completed four obstacles: crawling through a pit covered in barbed wire, scrambling over sludgy mud mounds, and traversing a stack of loose hay bales.

About two miles into the 12.5-mile course, Sengupta and his buddies came to obstacle number four: “Walk the Plank,” a 12-foot plunge into a freezing pit of muddy water, designed to “Test your fear of heights and cold all in one.” (A description on the Tough Mudder website warns, “Don’t spend too much time pondering your leap – Marines at the top of the platform will chew you out, or worse, push you into the freezing depths below.”)

“They sent us off in waves,” Muskin said. “We’d get to the top of the platform and they would count down for that line of people to jump. Then they would wait for the group below to get out of the way before sending in the next wave.”

Muskin, who was the second person on the team to take the plunge, said the water was cold. Sengupta went fifth: He leapt into the 12-foot-deep pit and never resurfaced.

His friends shouted frantically to rescue staff that Sengupta hadn’t come up. Muskin said it’s hard to tell how long he was underwater. “At that point, seconds feel like minutes, and minutes are hours.”

Sengupta was unresponsive when they finally dragged him to solid ground. EMTs started CPR right away and continued until an ambulance came. A childhood friend who saw Sengupta in the local hospital said that it was obvious Sengupta would not survive. “As soon as I saw him,” Daniel Gemp , “[I] absolutely knew there was no hope.”

Walk the Plank was closed for the rest of the day as the local sheriff’s department investigated the incident. Doctors at Virginia’s Inova Fairfax Hospital took Sengupta off life support the day after the event, saying that his brain had swelled due to a lack of oxygen, resulting in brain death.

Saturday’s accident might raise a critical red flag in obstacle racing. While most Tough Mudders stress over the much-hyped electric shock obstacles— and —just like in triathlon, water may prove to be the most deadly element.

Of the 42 triathlon fatalities that occurred between 2005 and 2011, thirty-one occurred in the water. In 2012, USA Triathlon (USAT), the sport’s governing body, released a in response to nine deaths that occurred during the swim leg of the previous year’s races. The report does not incorporate autopsy information, but USAT writes, “Available data indicates the swimming fatalities appear to be caused by episodes of sudden cardiac death.”

Pre-race jitters, caffeine, and cold water have routinely been blamed for such incidents. But something else may also be at play, both in triathlon and obstacle-racing water deaths.

“It’s not uncommon to hear stories of someone jumping into cold water and never coming up,” said , an expert on human responses to cold water at the University of Manitoba (profiled in ���ϳԹ��� in 2003). “A possible explanation is that if you completely submerge the body, and you get a cold shock response, you’re going to drown right there.”

“Cold shock” is defined as gasping and hyperventilation, followed by a possible cardiac response if the victim has underlying heart problems. The cold shock response can happen immediately upon jumping into water that covers the head. As , cold shock can occur in water temperatures as high as 77 degrees. Inhaling just five ounces of water is enough to drown you.

Giesbrecht said he’s not surprised to hear of Sengupta’s death: “I tell people when I give my lectures, ‘Never dive into cold water,’ for exactly that reason.”

USAT’s medical director, Dr. Andrew Hunt, said that, although competitors like Sengupta may seem in perfect health, it’s still possible for them to suffer an arrhythmia induced by cold shock. “One of the most common presenting symptoms of sudden cardiac death,” said Dr. Hunt, “Is sudden cardiac death.” Obstacle racers can be at a disadvantage in the water since they don’t generally feel the need to train for water features. “At least in Triathlon you go into it with the expectation that you have to swim 1.5 kilometers,” said Dr. Hunt. “I don’t think anybody goes into Tough Mudder with the expectation that they’re going to have to swim much.”

Unlike triathlon, obstacle races do not have a governing body that aggregates injury statistics from the hundreds of events held annually across the country. Obstacle racers concerned about their safety are left only with this promise from Tough Mudder:

“We are actively reviewing our safety procedures at Walk the Plank, as well as all other water obstacles on course. This review includes how we brief and manage water safety personnel, as well as volunteers. We will be making appropriate changes to upcoming events, including our Ohio event this Saturday.”

Sengupta’s Tough Mudder team is considering another Tough Mudder in the future to complete in memory of their friend. “But,” Muskin said, “Walk the Plank might be one of the obstacles we skip.”