The security company that owns the facility, G4S PLC, said the thieves had made off with "an unconfirmed sum of money" and added it would offer a large reward for information leading to their arrest and the return of the loot. The company did not give an exact amount for the reward.
One person was detained later Wednesday in a Stockholm suburb in connection with the robbery, but wasn't officially declared a suspect, police spokesman Christian Agdur told reporters.
Shortly after 5 a.m., the helicopter swooped down toward the cash depot and hovered over the building as the robbers hoisted themselves onto the roof in what police said was a carefully planned operation.
"What we know is that they forced down some kind of wall to get in," Agdur said. "We don't want to comment on how they did it." There were 21 staff members inside the building during the heist, but no one was injured, police said.
Investigators said the robbers wore masks and were believed to have carried automatic weapons and used explosives during the 20-minute raid. Witnesses reported hearing loud bangs during the heist.
Witness Bjorn Lockstrom told broadcaster TV4 he saw a gray helicopter hovering above the building for about 15 minutes.
"Two men hoisted themselves down," he said. "I saw when they hoisted up money, too."
A police commando team was seen trying to enter the cash depot in the Vastberga neighborhood with a battering ram.
Police later found an abandoned helicopter near a lake north of Stockholm, about 15 miles (25 kilometers) from the cash depot. Police spokeswoman Towe Hagg said the chopper was reported stolen and was believed to be the one used by the robbers.
A bomb squad was examining the suspected explosives left at the police hangar, preventing their helicopters from taking off.
"We've found what we believe is a live bomb to hinder our response," police spokesman Rikard Johansson said.
Britain-based G4S PLC is one of the world's largest security companies. The Vastberga facility stores cash that is transported to banks and other businesses in Sweden. G4s declined to say how much money was in the cash depot when it was attacked.
Sweden has seen a series of spectacular robberies in recent years. Last year a group of men broke into a mail processing center in Goteborg, paralyzing large parts of Sweden's second-largest city after spreading out spikes, burning out cars in different areas and leaving suspected explosive devices in the center.
In 2006, Goteborg's international airport was partially closed after masked men crashed through a gate and held up luggage handlers as they were unloading crates of foreign currency worth 7.8 million kronor ($1.1 million) from a passenger aircraft.
Four years earlier, robbers pulled off a similar heist at Stockholm's Arlanda Airport, when staff were loading foreign currency worth 43 million kronor onto an aircraft.
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