DENVER -- Two undercover officers eating lunch at a Vietnamese restaurant in southwest Denver thwarted a robbery Wednesday afternoon after a shootout with a masked gunman.

The gunman entered through the back door of the Ha Noi Restaurant, apparently trying to rob it, said Denver police spokesman Sonny Jackson.

Two undercover police officers were eating lunch at the time and saw the gunman come out of the kitchen.

"When he came out of the kitchen area, he was heavily armed, so that's when the officers engaged him and that's when the shots were exchanged," said Jackson.

Police did not say how many shots were fired, but one critically injured the gunman. He was taken to an area hospital and is listed in critical condition.

Police said the gunman underwent surgery and is expected to survive.

One of the officers was hit with a glass shard and has glass in his eye, police said. He was taken to the hospital and released later Wednesday evening.

Three other patrons in the Ha Noi restaurant suffered minor injuries and were treated at a hospital. Police didn't know if they were hit by glass or bullets that had ricocheted.

One person underwent surgery after the shooting and remains in the hospital. A second victim is in fair condition and the thrid was treated and released Wednesday night.

The incident was reported shortly after noon. The restaurant, located near Federal Boulevard and Tennessee Avenue, is part a strip mall with mostly Vietnamese shops and restaurants.

The area is home to many Vietnamese businesses. Police had to bring in interpreters in order to question some witnesses.

The robber appeared to have targeted the restaurant at random.

Other workers in the strip mall said news of the robbery put them on edge.

"It could have been easily us as them," said Diane Bushey.

Denver police said they would investigate the shooting to determine if the officers acted with appropriate force.