An armed gunman made off with an undisclosed amount of cash Monday after robbing the Brenton Bank at 723 24th St.

No suspects in the robbery were named at press time, but the FBI and Ames Police Department have formed a collaborative investigation effort.

At about 4:45 p.m., the suspect, carrying what police believe to be a semi-automatic handgun, entered the bank. The alarm was not activated, and the suspect fled the scene on foot, heading north through the North Grand Mall parking lot, according to police reports.

The suspect is believed to be a white male, about 6 feet tall and weighing about 200 pounds. He last was seen wearing a dark blue coat with white stripes on the arms and the collar turned up. He also wore a navy blue and white bandanna wrapped around his head.

Sgt. Randy Kessel, public information officer for Ames Police, said the FBI will be taking the lead on the case.

“As of 6 [p.m.] on Monday, the FBI took over the case,” he said. “Our investigators will be kept up to speed, however.”

Kessel said Ames police officers will be canvassing for potential witnesses to the robbery.

“We’ll be talking to bank employees and see what comes up from their memory after a day or two, seeing if they remember anything,” he said. “The main crux of our portion of the investigation will be around the area of the crime.”

Kessel said investigators were not sure whether the robber was working with an accomplice or had access to a getaway car.

“We can’t assume anything,” he said, “but the last visual contact bank employees had of the gunman was leaving the bank on foot. They did not see him getting into a vehicle.”

A sketch of the suspect will not be released because none of the witnesses got a definite look at the gunman’s facial features, Kessel said.

“At this time, it appears what we’ve got from firsthand witnesses and video tape that most of the facial area was covered, so a composite would not be an effective tool,” he said.

Kessel also said Ames Police was not notified of the crime until after it had occurred because bank employees were unable to activate the alarm.

Anyone who was in the area at the time of the robbery and who may have information about the incident is urged to contact the Ames Police Department at 239-5133.