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 having been murdered previously. Revealing himself as a Slitheen, Strickland 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 various kinds of vinegar over Sip Fel Fotch, causing him to burst, covering them with the remains of his body. (DW: Aliens of London / World War Three)