ADDISON, Mich. -

Police in Michigan are searching for the person who hacked a road sign and made it display a racial slur targeting Trayvon Martin.

Michigan State Police discovered the hacked construction sign early Monday morning on Interstate 94 near the town of Addison.

According to photographs taken by news viewers, the sign read, "Trayvon is (racial expletive)."  Troopers received several 911 calls about the sign by drivers who were passing by.

[Related: Martin shooting prosecutor mulls charges]

Troopers found the sign's control box unlocked when they arrived at the scene on Monday.  The keyboard used to type messages onto the sign was missing.

The message was removed from the sign.  Investigators are looking at surveillance video from the area to search for the hacker.