Police released pictures and surveillance video of the man on Thursday.
Upper Darby Police Superintendent Michael Chitwood said a sign "filled with threatening anti-Muslim rhetoric" was discovered at 5:15 a.m. Monday outside the Upper Darby Islamic Center and Masjid al Madinah in the 200 block of South 69th Street.
A similar sign was placed outside another facility operated by the same group nearby.
Chitwood said he considers the placement of the signs, both of which threaten harm to Muslims, a hate crime.
The suspect is seen in the surveillance video carrying and then placing homemade signs which appear to be wrapped in plastic.
He is described as a man 50-60 years of age, wearing a blue coat with a NASA logo, a red aviator or hunting style hat and blue jeans.
Anyone who has information about the case or who recognizes the man in the photos was asked to call Upper Darby Police at 610-734-7693.
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