DENVER -- Boulder police have arrested four men in connection with the robbery of a medical marijuana facility in Boulder Tuesday afternoon and are looking for more potental suspects.

The robbers took cash, two 20-gallon plastic tubs of marijuana and a surveillance system, said Boulder police spokeswoman Sarah Huntley.

She told 7NEWS the robbery of a medical marijuana store is relatively rare in Colorado, but says at least one potential shop keeper has now stepped forward with safety concerns.

"It's too early to tell if this is a trend," she said.

A woman who works at the facility told police that two men walked into the office and asked for an appointment. When she asked for a card to prove they could legally buy marijuana, they said they were actually here to rob the facility, Huntley said.

The men were seen running up a small hill to a waiting black Cadillac Escalade, she said.

No one was hurt in the robbery.

Officers caught up with the Escalade along U.S. 36 near 104th Avenue and took four people into custody for questioning, Huntley said.

The New Options Wellness Center has been in business for about four months. It is located at 2885 E. Aurora Ave.

None of the business owners that 7NEWS spoke with were even aware that pot was being sold from a small room in the back.

About 7 thousand Coloradoans have now been licensed to receive medical marijuana.