Here’s how it happened, according to a report in the tabloid daily Aftonbladet.

5.15am – A helicopter is witnessed above the roof of the G4S cash depot in Västberga in Stockholm by staff at the National Rail Administration (Banverket) offices directly opposite.

5.19am – Police receive a call about the robbery. Witnesses watch as the stolen white Bell 206 Jet Ranger helicopter lets out three or four passengers on the roof. Explosions are heard as the robbers enter the building while the helicopter hovers above the building.

Several sacks of what is presumed to be money are then seen raised into the helicopter on a rope.

5.25am – The first police patrol arrives at the scene in Västberga but are ordered not to act as the robbers continue to load sacks of money into the helicopter. They have been ordered to await the arrival of the National Task Force.

5.35am – With the police looking on the helicopter lifts off from the roof of the building and heads north. The robbers had lain caltrops on the road routes out away from the building in order to hinder the police.

The National Task Force enters the building.

5.49am – The police helicopter station at Myttingen on Värmdö is notified but pilots can not lift off as a suspected bomb is found in front of the helicopter hanger.

7am – The police confirm that none of the 21 G4S staff were hurt in the attack.

7.37am – Explosives are found in the building and police extend the cordon around the depot.

8.15am – The helicopter is found in woodland near Skavlöten in Arninge north of Stockholm.

8.42am – The police escort the G4S staff out from the cash depot.

1pm – police dispose of the suspected bomb at the helicopter station on Värmdö with the help of a water cannon.

3pm – Police hold a press conference where they classify the crime as an extraordinary event and are thus able to call on the resources of police forces from across the country.

By 6pm in the evening the police had arrested two men in connection with the robbery as they combed the Stockholm underworld for clues to what is described as a well-organised professional heist.

Criminology professor Leif G W Persson said on Wednesday that the cash depot could have housed up to a billion kronor ($146 million) in cash.

Media reports on Thursday indicate that a mafia boss from the Balkans could be the brains behind the robbery.

It is also reported that Stockholm police had previously received information that a helicopter heist was being planned in the area but that the National Task Force had been conducting surveillance at the wrong depot, in Bromma in the north-west of the city.