It happened around 9:45 a.m. at the TruMark Financial Bank branch at 1931 South Broad Street in Philadelphia.
Investigators say the man entered the bank, brandished a silver-colored semi-automatic pistol, and demanded cash from a teller.
After obtaining an undisclosed amount of cash, the subject fled the area of the bank on foot and was last seen headed southbound on Broad Street.
The suspect is described as a black male, mid to late 30's, 6'0" to 6'3" tall, 200 to 250 pounds, heavy build, wearing a blue hooded sweatshirt with a zipper front and the hood up, a baseball cap under the hood, a surgical mask over his mouth, grey shorts, and a surgical glove on his right hand, which was the hand he was holding the gun in.
The man is considered armed and dangerous.
Anyone with information is asked to call the FBI at 215-418-4000, or the Philadelphia Police Department.
