Gardaí are probing the background of several Bank of Ireland employees as part of their investigation into the state's biggest ever bank heist on Friday and are satisfied it could not have been carried out "without insider information".

Some €4m of the estimated €8m stolen has been recovered. Gardaí acted swiftly after being informed bank official Shane Travers, whose girlfriend and members of her family were being held hostage, had left the premises after being forced to withdraw cash from Bank of Ireland on College Green early on Friday morning.

Following a series of raids and arrests that continued until late on Friday night, seven people remained in garda custody yesterday.

Criminals from an inner-city crime gang were responsible for the heist. A relative of a well-known gangland figure is in custody. This man, arrested with another criminal in a car close to the Bridewell garda station, was not found in possession of any of the stolen money and there is not a significant amount of evidence against him. There were 15 searches of houses and cars in Dublin as well as raids in Longford and Westmeath late on Friday but none of the arrests or money seized occurred outside the capital. It is understood at least three of the people in custody are out on bail for other offences.

The six men and one woman in custody range in ages from early 20s to late 30s.

The majority of the recovered money was found in a car, which was intercepted by armed detectives following a chase through Blanchardstown. Another car was stopped in Phibsborough and a substantial sum was found.

The man believed by detectives to have masterminded the ambitious heist is facing charges in relation to another high-profile kidnapping and robbery.

This weekend, gardaí shifted the focus of their investigation onto the collapse of internal bank security and began probing the background of bank employees. "The criminals had a lot of information about who was working in the bank that morning. It had to be carried out with a degree of insider information," according to a source.

It is understood detectives have been able to link one of the bank's employees to the criminal gang who carried out the robbery. Follow-up searches for the stolen money, which is traceable, will continue today and gardaí "anticipate more arrests. We wouldn't be surprised if they try and dump it or even burn it."

A source said the criminals might have had a better chance of getting away had they only stolen €1m. "Their greed might well have been their downfall," he added.

Early-morning staff at the branch agreed to help Travers (24) fill four plastic bags with cash after he had shown them a photograph of three hostages being held at gunpoint.