Boston police were searching Wednesday for a suspect who allegedly threatened a mother and son with a knife earlier this week, stole cash from the woman’s purse, and then exposed himself to them before fleeing. It occurred around 6:30 a.m. Monday in the 200 block of Paris Street in East Boston, according to a police statement.

Police said the 43-year-old woman was leaving home with her 10-year-old son when the suspect, described as a muscular white man with green eyes and yellow hair, about 6 feet 2 inches, entered the stairwell area of the building, and showed the pair a folding knife.

The suspect, who was wearing all black, including a mask, then took cash from the mother’s purse and “indecently exposed himself,’’police said.

A neighbor heard the commotion and ran down the stairs, yelling at the suspect, who fled to a dark-colored sedan, believed to be a Toyota Corolla or Camry. The license plate included 5FX.

Danny McDonald can be reached at daniel.mcdonald@globe.com.