The robbers subdued the workers with duct tape and locked them inside the vault after the raid, Caruso said. Some media outlets reported that the robbers made off with $4 million and $5 million. Caruso said he had no immediate details of how much was taken.
Authorities found the armored vehicle -- a specially modified van -- about 90 minutes later less than two miles away.
"You don't see anything like this every day," Caruso told reporters. "It's certainly alarming, to say the least. It was carefully planned. They knew what they were doing. They didn't appear to have any intent to hurt anybody, which is an important key here."
Caruso waved off a reporter's question as to whether there was any indication an employee of the business may have taken part in the heist, given the knowledge the suspects appeared to have about the business' routine. That's "totally unknown," Caruso said.
The FBI was assisting in the investigation, an agency spokeswoman said, declining additional comment beyond what police released.
Caruso said the four bandits waited for one of the business' employees to come to work, then rushed in behind him, disarmed and subdued him before waiting for a second employee to arrive, apparently knowing it took two workers to separately punch in access codes to open the vault.
Once inside the vault, Caruso said, the suspects bound the workers with duct tape, loaded up the cash in containers and fled, locking the two victims inside the vault unharmed.
Caruso said two of the suspects had semiautomatic pistols, one had a rifle and the fourth had an unspecified type of firearm. Bullets found scattered outside the business were from the victim's handguns that the suspects stole and emptied, Caruso said.
Police planned to look at some surveillance video from the area, hoping it offered clues to the suspects' identities, Caruso said.
Cincinnati-based ATM Solutions replenishes cash in ATMs and maintains the machines for banks and other customers in Missouri, Illinois, Indiana, Ohio, Kentucky and Tennessee. A message left Monday with the company was not immediately returned.