Several powerful explosions were heard in the early hours on Wednesday coming from the G4S security company's facilities in Västberga, south of Stockholm. Among other things, the building houses a cash depot.
Police received a call about the incident at 5.19am.
The chief of police for the southern district in Stockholm, Ulrika Lönngren, who has her office in Västberga, reported hearing several loud explosions and a helicopter hovering above the depot.
The helicopter later left the area and headed north, said Lönngren.
“The helicopter landed on the roof. Then they entered the building by breaking some windows. Next, several explosions were heard from within the building, and then they were seen loading things into the helicopter and lifting off,” she told the TT news agency.
The entire area surrounding the G4S facility has been cordoned off and police have set up a command centre at a nearby petrol station, where a large number of officers, police dogs, and emergency vehicles have gathered.
An officer on the scene told TT that the robbers rappelled down from the helicopter as it hovered above the roof and then entered the building using sledgehammers or something similar to break through the roof.
Caltraps - spikes spread across the road to burst car tyres - had been placed near the facility in an attempt to complicate efforts by police to approach the scene.
According to the southern district police, Sweden's National Task Force, a paramilitary tactical unit within the National Criminal Investigation Department (Rikskriminalpolisen) is involved in the investigation.
The entire operation lasted about 20 minutes, but it remains unclear if the robbers made off with anything, police spokesperson Björn Engström said.
Police helicopters located on nearby Värmdö were unable to be used in response to the robbery, prompting suspicions that they had been sabotaged.
But Engström said that the helicopters hadn't been damaged. Rather, they found a bag marked "bomb" near the helicopters forcing officers to inspect the bags and the helicopters before attempting to take off, the Svenska Dagbladet (SvD) newspaper reports.
Police have confirmed the helicopter used in the heist, a Bell 206 Jet Ranger, was stolen overnight from a helicopter hangar near Norrtälje north of Stockholm.
Later Wednesday morning the helicopter was found near Arninge about 30 kilometres north of the crime scene.
According to Lönngren, there were between 10 and 20 employees working in the cash depot at the time of the heist, many of whom were involved in counting money. They are now being interviewed by police.
The manager of the depot, Roy Gierde, told the Aftonbladet newspaper that there is a high likelihood the robbers were caught on film.
"We have cameras everywhere," he told the newspaper.
G4S is a security and cash handling company with operations in over 110 countries and 530,000 employees worldwide.
The company employs about 1,000 people in Sweden, where it handles large amounts of cash for businesses and banks.
The Stockholm-area depot holds cash for several banks and shops in and around the Swedish capital.