A rider and two spectators were killed on the final day of the centenary Isle of Man TT races, organisers said on Friday.
A spokesman said the name of the rider, competing in the Senior TT, was being withheld pending notification of next of kin.
Organisers said the incident at the 26th milestone on the mountain course involved four people apart from the rider, who was confirmed dead from his injuries at the scene along with a spectator.
Another died after admission to the island's Nobles hospital and two were injured.
An investigation was under way into the causes of the accident.
The deaths, on a circuit that before Friday had claimed 223 lives since the inaugural Tourist Trophy race of 1907, overshadowed what had otherwise been a memorable close to the week-long festival of road racing.
Briton John McGuinness won the senior TT to become the first rider to lap the island circuit at an average speed of more than 130mph.
McGuinness's 13th TT win, after his triumph in the delayed opening Superbike TT on Monday, made him the third most successful rider in the event's 100 year history.
His fastest lap was timed at 130.354 mph, stripping 51 seconds off the previous record.