CABIN JOHN, Md. -- Officials are investigating what they describe as a robbery attempt in Cabin John, Md.

Investigators said a man entered a post office on Macarthur Boulevard just before 8 a.m. Wednesday, carrying a weapon that witnesses said looked like a small machine gun.

The man, witnesses said, corralled post office patrons and demanded money. But the patrons said they didn't have any money. The robber responded, "This must be your lucky day," witnesses said. No one was hurt in the incident..

Police said the man was dressed all in black, including a ski mask, and was wearing goggles and a helmet. They said he unsuccessfully tried to rob several people.

The man was last seen riding a bicycle down Macarthur Boulevard toward the beltway, police said.

Wednesday afternoon, postal inspectors went door-to-door looking for witnesses and evidence.