Sources say that officers are "very confident" that the hammer was used by the gang when they carried out the €4,000 robbery at the Lordship Credit Union in Co Louth last month.
Detectives are now examining the hammer for vital fingerprint or DNA evidence as the investigation into the savage January 25 murder continues.
Details of this aspect of the investigation, along with other leads, will be outlined at a press conference this afternoon.
Two senior officers – Assistant Commissioner Kieran Kenny and Superintendent Gerard Curley – will reveal the new lines of inquiry before renewing their public appeal.
Two men arrested and questioned last Thursday about the stolen getaway car were released over the weekend.
A file on them will now be prepared for the DPP.
They are not suspected of direct involvement in the murder but are thought to have knowledge of the criminals who stole the navy-coloured VW Passat that was used by the gang.
It is understood that the PSNI have quizzed a number of the gang believed to have been involved in the detective's murder, including a young woman who provided an alibi for her boyfriend.
This man is suspected of being at the scene on the night of the murder. He also faces serious charges in the north and is on bail. He is also on bail in the south in relation to charges of assault.
He is the chief suspect for organising and participating in a raid on the Lordship Credit Union in 2011 which gangsters escaped with €62,000 in cash.
It is understood that Detective Donohoe personally questioned the suspect on a number of occasions. Two brothers from Crossmaglen, Co Armagh, are also in the frame.