PHILADELPHIA (AP) -- Philadelphia police are investigating what they suspect is a sick Halloween prank: someone who left photocopies of letters from the infamous Zodiac Killer on cars.

The Zodiac Killer is a still-unknown serial murderer who killed five people and injured two others in the Bay area in the late 1960s.

The killer left a series of taunting notes to Bay Area media outlets, and Philadelphia police say copies of one such letter were distributed on Halloween.

Police Capt. George Kappe tells WTXF-TV that the there are no threats that lead police to believe people might be in danger.

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