Officials say the city's website was hacked today, though users were able to access most online services.

Federal authorities and Chicago police are investigating the attack, according to a source familiar with the incident. Authorities are also exploring whether the attacks are linked to a threat to hack NATO's website in Europe.

Hackers claimed credit for bringing down the cityofchicago.org site, according to Twitter feeds this morning. The cityofchicago.org appeared to be back to normal by 2 p.m.

A group using the name antis3curityops sent out a message on Twitter at 6:17 a.m. directing people to “fire” on chicagopolice.org and chicagoseargeants.org.

A Twitter user who claims affiliation with Anonymous, an international hacker community, tweeted a little later "Tango Down," with a link to cityofchicago.org -- shorthand used on the Internet to indicate a site has been hacked or targeted. That site was also unresponsive as of noon.

The group antis3curityops posted a video saying it was "actively engaged in actions against the Chicago Police Department," and it encouraged "anyone to take up the cause and use the AntiS3curityOPS Anonymous banner. For those able, chicagopolice.org should be fired upon as much as possible. We are in your harbor Chicago, and you will not forget us."

Referring to clashes between police and protesters during marches in the Loop Saturday night, the video said "let us unite and show the violent Chicago police and the government big brother tactic’s (sic) that we are not gonna take this."

A Twitter account claiming to represent Anonymous cited "violation of humanrights" as a reason for the cyberattack against the police site.

hdardick@tribune.com