A runner collapsed and died while taking part in the Brighton Marathon on Sunday 14 April.
The victim, described locally as a 23-year-old man, had completed around 16 miles of the 26-mile long course when it happened.
He suffered a suspected cardiac arrest near Grand Avenue in Hove, during the run by the seaside.
An off-duty police officer rushed to him and provided help on the road.
Emergency services treated him at the scene before taking him to the Royal Sussex County Hospital. But he was pronounced dead three hours later.
Next of kin have been informed.
The tragedy overshadowed what had been a joyous day for lots of the more than 9,000 people taking part, watched on by cheering crowds.
England wicketkeeper Matt Prior fired the starting gun which set runners off, on Sunday morning.
One runner who completed the course called the mood in the finishing area "jubilant".
"The crowds and music were great, and really kept things going."
Race director Tim Hutchings said: "It's tremendously sad. I can confirm that a competitor collapsed and later died in the Brighton marathon this morning.
"We have rigorous emergency plans here in Brighton, much of the best practice we've taken from events as high calibre as the London marathon.
"The individual was treated at the scene by senior doctors, nurses and paramedics.
"He was then taken by ambulance to hospital but despite receiving an awful lot of attention he was later declared dead at the Royal Sussex County Hospital.
"Obviously this is dreadfully sad news for the entire Brighton marathon team and we pass our condolences on to all his family."
He added: "At the moment there is rather a sombre mood among the marathon team here. We're thinking most of all about the [runner's] family."
Kenyan runner Dominic Kangor won the race in a time of 2hrs 10mins 46sec. Close in second place was Bernard Rotich, who was only five seconds behind. Robert Mwangi took the final podium spot with his run of 2:11:26.
Jon Pepper was the highest-ranking British runner in the field, with a time of 2:20:30.