A gunman robbed a Riverhead drugstore Thursday and eluded a multiagency dragnet, police said.

The robber showed his handgun at Walgreens, 1196 Old Country Rd., while demanding money from an employee at the register area just after 7 p.m., Riverhead police said.

He ran away after getting cash, police said.

Officers from several agencies, including a K-9 unit from the Suffolk County Sheriff’s Office, searched for the robber, police said.

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The suspect appeared to be 40 to 45 years old, about 5 feet, 8 inches tall and had an average build, police said.

Police ask witnesses or anyone with information to call 631-727-4500, ext. 332.