Shortly after 5 p.m. Thursday on a main Georgetown thoroughfare, a man entered an enclosure in which cash may be withdrawn from an ATM.

A robber apparently slipped behind the victim inside the enclosed area near 30th and M streets NW and placed an object against his back.

The robber claimed to have a gun and demanded money, police said.

The incident, although it occurred in a place generally regarded as safe, underscored calls for caution in making ATM withdrawals.

Thursday’s incident behind the glass door of the SunTrust Bank ATM vestibule apparently did not go unnoticed.

It was spotted, D.C. police said, by officers who “recognized this as a robbery” and entered the vestibule immediately.

Police said Bruce Allen Smith, 50, of Culpeper, was arrested and charged with robbery while armed.

Five days earlier, just before 6 p.m., on another street generally regarded as safe, robbers took a cellphone from a woman walking in the 2400 block of K Street. Police said they arrested three juvenile suspects.