A post office was robbed last week in Alexandria, a crime of a type that is relatively rare in the Washington area and around the nation.

A letter carrier was shot and killed late last year while delivering mail in the Cheverly area, but the motive in that shooting has remained unclear.

The robbery occurred Friday in the 1900 block of Mount Vernon Avenue about 4:15 p.m., the Alexandria police said. The facility is in the heart of the residential Del Ray area and is about four blocks from Route 1.

A masked man entered the post office and implied that he was armed, the police said. He demanded money, was given cash and then fled, the police said.

No one was injured.

It appeared that the robbery was one of only a handful in the Washington area during the past year in which money was taken from a post office.

One such robbery was reported Aug. 8 at a post office on Hamilton Street in the Hyattsville area of Prince George’s County.

A suspect was arrested in connection with that robbery and three others. The weapon used was a paintball gun, the D.C. police said.

On July 18, an armed robbery was reported at the Seven Corners post office in the Falls Church area of Northern Virginia.

In that robbery, one man entered the facility, vaulted over the counter, and demanded money from a postal worker, according to the Postal Inspection Service, the postal agency’s law enforcement arm.

An accomplice stood near an exit and displayed a handgun, the service said.

The two then fled on foot.

Armed robbery of a U.S. post office is a federal crime that carries a penalty of up to 25 years in prison.

Alexandria police said they were investigating Friday’s robbery with the Postal Service.

They said the service was offering a reward of up to $25,000 for information leading to the robber’s arrest and conviction.

The robber was described as a black man between 5-foot-8 and 5-foot-10 with a thin build. He wore a black mask, green military-style jacket and red or brown utility pants, the police said.

A television news account of the armed robbery of a post office in East Pittsburgh last November quoted a postal inspector on the frequency of such events. He said the do not occur very often.

Post offices are generally located in busy areas, and few keep night hours.

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