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This July 18, 2013, shows the Detroit skyline from Grand River in Detroit. Monday, Aug. 19, 2013, is the deadline for a host of banks, bond insurers, two employee pension systems and others standing to lose big if a federal judge declares Detroit insolvent to legally file their objections to the largest municipal bankruptcy in U.S. history. (AP / Carlos Osorio, File)

AUBURN HILLS, Mich. _ Police are investigating after motorists travelling through a Detroit suburb were stunned to see a pornographic video playing on an electronic billboard.

Auburn Hills police say the video played on the billboard along Interstate 75 near Michigan highway 59 for about a half-hour Saturday night before police contacted the company that owns the billboard and the images were removed.

Police say multiple people called 911 after seeing the explicit video.

Motorist Chuck McMahon tells WDIV-TV he saw the video and wondered if the billboard was advertising for a strip club.

Police are investigating how the images ended up on a billboard, including whether someone hacked into the electronic sign's system.

Dr. Justin Kammo also saw the video. He tells WXYZ-TV he immediately assumed that ``someone had hacked it.''