Chicago websites under cyber attack?

(CBS News) CHICAGO - Authorities are investigating possible DOS attacks against government websites in Chicago, site of the NATO Summit, CBS Station WBBM reports.

The Chicago Police Department website, cityofchicago.org/police, was down temporarily Sunday, but was functioning again in the afternoon.

The main portal to cityofchicago.org was not accessible for part of Sunday morning, but appeared to be functioning again by about 12:40 p.m.

"We are aware of the potential issue of the City of Chicago website and are working with the appropriate federal authorities to address the situation," Office of Emergency Management and Communications spokesman Pete Scales said earlier Sunday.

Hackers may also be attacking NATO-related websites, but details were not available, sources told WBBM correspondent Jay Levine.

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A posting on the website cyberwarnews.info explains the hacking group called Anonymous started "an operation toward the Chicago police department and its websites.

"We are actively engaged in actions against the Chicago Police Deptartment [sic], and encourage anyone to take up the cause," the group said. "We are in your harbor Chicago, and you will not forget us."

The group explains the attack is in response to the NATO Summit being held in Chicago. The summit officially opens this afternoon and runs through Monday.

Thousands of protesters have been filling the streets of Chicago all week to protest NATO's mission.

The biggest rally yet is planned for Sunday afternoon from Grant Park to near McCormick Place, where leaders are gathered.