STOCKHOLM (Reuters) - Swedish police said they were holding one suspect after armed robbers used a helicopter to stage a spectacular raid on a cash storage unit on the outskirts of the capital on Wednesday. The gang landed a helicopter on the roof of a cash storage facility belonging to Anglo-Danish firm G4S in Vastberga, just south of Stockholm, and made their way into the building through a window, police said. Witnesses reported several explosions. The gang then loaded up the helicopter and flew off. The attackers used helicopters and explosives to enter the premises and they escaped with an unconfirmed amount of money, G4S said in a statement. A substantial reward is being offered for information leading to the arrest and conviction of those responsible and recovery of the stolen money. Police said they were questioning one person suspected of being involved in the heist but that the person had not been charged. They gave no further details. It was well-organised, it was well-planned, police spokesman Christian Agdur said at a press conference. Nobody was injured in the raid though several of the staff at the warehouse were left shaken up, police said. The helicopter was later found abandoned north of the city and police dogs were being used to search the area. A police helicopter in the Stockholm area had been prevented from taking off in pursuit of the robbers by a suspicious bag close to the aircraft. A chopper had to be called in from Gothenburg, some 400km away, police said.