DULUTH — Duluth police have arrested the owner of a downtown head shop and his son as part of a long-running investigation into synthetic drugs.

Police say they searched the store called the Last Place on Earth on Friday and took several items, including suspected controlled substances, cash, documents and a gun. It’s the third raid at the downtown store in a year and a half.

Owner Jim Carlson and his son, Joseph Gellerman, were jailed on suspicion of fourth-degree sale of a controlled substance. Formal charges are expected next week.

Carlson has long insisted the products he sells are legal and has fought the city’s efforts to shut him down.

Carlson, Gellerman, and two others were indicted on 54 federal counts last year. That trial is set for September.