Search warrant records filed Tuesday showed federal agents also seized computers, correspondence and a handwritten note among dozens of items at the Annapolis apartment of 88-year-old white supremacist James von Brunn, The Baltimore Sun reported.
He is charged with fatally shooting Special Police Officer Steven Tyrone Johns at the museum June 10.
Von Brunn, shot by officers who returned fire at the museum, remained in critical condition at a Washington hospital. His initial court appearance is set for Monday.
In an affidavit, FBI Special Agent Christy Shaffer said agents had to search the apartment to determine whether "the gunman was working alone and whether additional targets were established" to assess "potential immediate threat."
Von Brunn pays about $400 a month to rent a room in an apartment with his son, Erik von Brunn, and the son's fiancee, Brandy Teel, The Sun reported.
In a statement to ABC News, Erik von Brunn, condemned his father's views.