Strickland

Assistant Commissioner Strickland was the senior police officer who visited Jackie Tyler after she informed the authorities of the Ninth Doctor's presence. In fact, Strickland was Sip Fel-Fotch Passameer-Day Slitheen, a disguised member of the Family Slitheen. The real Strickland had been murdered. Strickland revealed himself as a Slitheen and attacked Jackie and Mickey Smith in Mickey's flat. The Doctor, Rose Tyler and Harriet Jones assembled enough clues about the Slitheen's physical make-up for the Doctor to deduce their weakness. Jackie threw vinegar over Sip Fel Fotch, causing him to burst and coat them with the remains of his body. (TV: Aliens of London / World War Three)