Stockholm  - Prosecutors Wednesday asked a court to remand four men in custody on suspicion of involvement in a daring heist last week when thieves used a helicopter to raid a security firm in Stockholm.

Among the four was a 34-year-old suspect alleged to have flown the stolen helicopter used in the raid last Wednesday.

Two other men, aged 21 and 31, were released Tuesday, but police said they were still under suspicion.

The custody hearings were due later Wednesday at the Sodertorn district court, and there was risk that the investigation would be harmed if the suspects were released, the prosecutors maintained.

"Everything suggests considerably more people were involved in the very well-organized robbery," prosecutor Bjorn Frithiof said in a statement.

The criminals used a stolen helicopter and explosives in the early morning raid at the G4S cash depot in the suburb of Vastberga.

They escaped with an undisclosed amount of cash, having given themselves a headstart by planting a fake bomb near the police helicopters that would have been used in any pursuit.

The criminals' helicopter was later found abandoned in a field north of the Swedish capital. G4S has offered a reward of 1 million dollars for information leading to arrests.

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)