The body of the 18-year-old was found in a burnt out red Mazda 6 car at Fairview Road in Galliagh on Saturday, June 1, 2019.
At first it was thought that the car had caught fire as a result of a collision.
But the results of a post-mortem examination showed that Caoimhin was not seriously injured as a result of the crash.
Police believe there were other people in the car who made off after the collision and left Caoimhin behind in the car.
The officer leading the inquiry into Caoimhin’s death, Detective Inspector Michael Winters said in a statement: “Our enquiries have been ongoing and this is very much a live investigation.
“However, we would like to hear from anyone who may be able to help with our enquiries.
“The body of 18-year-old Caoimhin Cassidy was found inside a red Mazda 6, which had crashed on the Fairview Road during the early hours of Saturday, June 1, 2019.
“Initially it was believed that the car had caught fire as a result of the collision.
“However, the results from the post mortem examination subsequently told us that Caoimhin was not seriously injured as a result of a crash, and was most likely still alive when the blaze started.
“If you can cast your mind back and remember anything about Caoimhin’s movements, or those of the red Mazda 6, on the evening of Friday, May 31, then please get in touch.
“Any detail, no matter how small it might seem, could be invaluable.
“Contact detectives at Strand Road on 101, quoting reference number 316 of 01/06/19.
“Alternatively, information can be provided to the independent charity Crimestoppers on 0800 555 111, which is 100% anonymous and gives people the power to speak up and stop crime.”