Sources told the Irish Examiner on Friday that Detective Adrian Donohoe may have been gunned down because he recognized one of the gang. Donohoe and another detective were acting as an armed escort for a cash delivery at the Lordship Credit Union outside Dundalk in County Louth on Jan. 25 when he was killed.
A stolen Volkswagen Passat believed to have been used in the attempted robbery was found burned out on the other side of the border in County Armagh.
An investigator said that "staunch Republicans" have been in touch with police offering to help with the investigation, something he called "extraordinary."
Two brothers from the Dublin area were detained on suspicion of being involved immediately after the killing. They have since been released.
Sources told the Examiner police believe there are five core members of the gang, all in their late teens or early 20s. Two are brothers.
The credit union was robbed in 2011 during a cash delivery. On that occasion, the only escort was an unarmed police officer and the thieves made off with thousands of dollars. Police say the same group may have tried a second robbery.
Thousands of police officers attended Donohoe's funeral service Thursday in Dundalk. His wife and a brother are also officers.
Notable deaths of 2014 [PHOTOS]