Police say the man, who's not yet been named, died at around 6.40pm on 18 September.
Police say the 66-year-old died in a single vehicle accident on the track to Fort Clonque, at Alderney's south-western tip.
They say as investigations are carried out there will be a police presence in the area and ask people to stay away.

Guernsey venues work together to support Vale Earth Fair
Royal Guernsey goats on show at Government House
Grassroots Guernsey football clubs get matchday ball donation
New operator sought for Guernsey's Digital Greenhouse
One fifth of Year 1 schoolchildren in Guernsey overweight
Sark Shipping told to get its finances in order
Reliability of Alderney airlinks to be reviewed
Guernsey's final summer show cancels the fireworks display