Gunmen associated with a Russian crime gang are the main suspects in the murder of gangland enforcer Gerard 'Hatchet' Kavanagh.
Dubliner Kavanagh was gunned down in a hail of bullets as he drank in an Irish bar on Spain's Costa del Sol at the weekend.
Gardai are working with Spanish police in a bid to pinpoint the identity of the balaclava-clad killers who carried out the ruthless shooting.
And they are investigating three possible motives for the murder.
But last night officers said the most likely organisers of the hit were members of a Russian outfit involved in a deadly feud with Ireland's top drug trafficker, Christy Kinahan, over payment for a cocaine shipment.
The Russians are claiming that they supplied Kinahan with a multi-million euro consignment of drugs as part of an underworld deal, in which Kavanagh was said to have been involved as a go-between during negotiations.
But it is believed Kinahan has denied that he ever received the shipment and has refused the Russian demands for immediate payment.
Investigators say the row between the two groups over the money has become increasingly tense and they think Kavanagh was murdered to put pressure on the Kinahan gang.
They feel the dispute in Spain provides the background to the shooting, rather than a row between the Kinahan gang and one of their former associates over a debt owed in Dublin.
Gardai are keenly watching developments in an ongoing row between members of Kinahan's gang and the former associate, which resulted in attacks on a car and a house in south Dublin in recent weeks. Officers said that "in the good times" drug shipments were supplied from Spain and payment made after they had been sold on the streets.
But a fall-off in the cocaine trade, in particular, because of the recession, as well as the garda seizure of a number of consignments and poor "laundering" investments, has meant that traffickers based here have fallen into serious debt with their international suppliers.