The lead investigator is going back on the witness stand in the trial of one of the two key figures in a prostitution scandal centered at a Zumba studio in coastal Maine. Lawyers for an insurance agent charged with promoting prostitution will resume their cross-examination Monday of Kennebunk police officer Audra Presby and begin calling additional witnesses. The defense contends Presby retaliated against defendant Mark Strong Sr. by targeting him because he was looking into unprofessional conduct by Kennebunk officers. Videos played for jurors indicate Strong watched paid-sex from his computer 100 miles away in Thomaston and discussed operational details with Zumba instructor Alexis Wright. The 57-year-old Strong acknowledged co-signing for Wright’s studio lease but contends he didn’t profit from her activities. Wright will be tried later.