KANSAS CITY, Mo. -- Police said three men used an assault rifle to rob a popular Zona Rosa restaurant Sunday night.

Investigators said the men came through an unlocked back door of Qdoba Mexican Grill and demanded money.

The restaurant was closed, and the only people inside were employees who were there to clean up, KMBC's Chris Nagus reported.

"It's weird, because I don't see why you rob a Qdoba," customer Elliott McCalley said.

"It's crazy. I come over here every day for lunch," customer Leah Conroy said.

Conroy said she's surprised that robbers would target a restaurant in the popular shopping district.

"Not in Zona Rosa -- maybe down in the ghetto, but not here," Conroy said.

Qdoba was open for business on Monday, but they opened a little later than usual, Nagus reported.

Zona Rosa issued a statement that security is a top priority in the area, and they believe the robbery is an isolated incident.

No arrests have been made. The suspects were described as three white men between 18 and 25.

Anyone with information in the case can call the TIPS Hotline at 816-474-TIPS.