The daring pre-dawn heist stunned police in the Swedish capital, who were unable to deploy their own helicopters to the scene because suspected explosives had been placed at their hangar.
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.
Shortly after 5 a.m. (0300GMT, 11 p.m. EDT Tuesday), the robbers flew over the building, jumped onto its roof and smashed a window to enter the cash depot, police spokeswoman Ulrika Lonngren told broadcaster SVT. There were staff inside the building, but no one was injured, she said.
Witnesses reported hearing loud bangs during the heist. G4S said in a statement that the robbers had used explosives during the heist, but did not elaborate.
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 Swat team was seen desperately 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.
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.
