The armed robbery occurred as an armored car arrived at Garda Armored Car Service at 1650 Northpoint Parkway around 10:45 p.m., Lt. Paul Henry said.
It's unknown how many suspects wearing masks were involved.
Two employees of the business who were present at the time were unharmed.
The suspects left the business with an undisclosed amount of money and closed the steel doors behind them, leaving the victims inside, Henry said.
The FBI also is investigating the armed robbery.
The Sonoma County Alliance has offered a $2,500 reward for information that leads to arrests.
Anyone with information should call Detective Ryan Corcoran or Sgt. Steve Fraga at (707) 543-3590.