Alleged drug dealer Allen (24) was murdered by a group of individuals at his home in Buncrana on February 9 2012. Republican Action Against Drugs later claimed responsibility for the killing.
The 24-year-old, who was the subject of intimidation in Derry before leaving the city, was shot through a window on the Links View estate in Buncrana after a group of men tried to break down the front door.
It's been three years since his death and an appeal has been launched by Crimestoppers to gather information on the killing.
"While up to a dozen individuals were involved in planning and carrying out of Andrew's murder, police understand that some of these did not intend for the events that unfolded to lead to his death," investigators said today.
"An appeal is now being made directly to these individuals or any other persons involved in the planning or carrying out this murder to take this opportunity to come forward."
Police are also hoping that other witnesses resident in both Buncrana and Derry will come forward with information on the callous murder. Thousands of leaflets will be delivered through doors in the coming days.
"Citizens who wish to assist have the option of giving this information anonymously to the Crimestoppers Line on 1800 25 00 25 in the Republic or 0800 555 111 for the North."
Calls to Crimestoppers are anonymous, they said, and callers do not need to leave their name of address. You may also receive a reward for information which significantly helps the investigation.