A You Tube video posted on cyberwarnews.info announced what it said was "an operation toward the Chicago police department and its websites."
There was no official comment from the city; however, the Sun-Times said law enforcement officials were looking into what caused the Web site to crash Sunday morning.
Leftist activists have been focused on Chicago in recent days because of the high-level NATO summit that opened in the Windy City Sunday.
The You Tube video apparently came from a group calling itself AntiS3curityOPS, which cheered on demonstrators who had faced off with police in the streets and urged supporters to launch their own computer attacks against the CPD Web site.
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