Stockholm  - A reward of up to one million dollars was posted Friday by the security firm in Sweden which was the victim of this week's elaborate helicopter heist of a cash depot near Stockholm.

Criminals used a stolen helicopter and explosives early Wednesday to raid the cash depot in Vastberga. The helicopter was later found abandoned in a field north of the Swedish capital.

The amount of stolen cash has not been revealed.

The helicopter remained one of the main leads in the investigation, although the suspects were still at large.

The reward was posted for information leading to the arrest and prosecution of the culprits, or the return of the booty, Security services company G4S said in a statement.

Police were continuing to analyze hundreds of tips as part of the probe. In addition, police forces in other countries had been contacted via Interpol and Europol.

About 100 police officers were part of the investigation - making it likely the largest in Sweden since the September 2003 murder of Foreign Minister Anna Lindh.

Aviation experts said the pilot who flew the helicopter was likely very skilled, citing the pre-dawn flight to the depot building and the location of landing site close to high trees. (dpa)