The five online news sites of Embarcadero Media, including PaloAltoOnline.com, were hacked Thursday night at about 10:30 p.m. but are now back online.

The sites, which provide online content for the Palo Alto Weekly, The Almanac and Mountain View Voice in the Mid-Peninsula and the Pleasanton Weekly in the East Bay, were attacked by someone who used an image of Guy Fawkes, the icon of the activist group Anonymous.

Embarcadero Media took down the websites – PaloAltoOnline.com, mv-voice.com, almanacnews.com, pleasantonweekly.com and danvillesanramon.com – while working to restore them.

There is no indication that the email addresses and passwords of readers who are registered users of the company's websites were hacked, but the perpetrator did have access to those data files,which could have been taken, according to Embarcadero President and CEO Bill Johnson. He advised readers who are registered users to change their passwords and, if they use the same password on other sites, to change all of them.

The company does not store any credit card information on its computer systems, so that data is not at risk, he said.

The hacker posted a message that indicated disgruntlement with The Almanac, which covers Menlo Park, Atherton, Portola Valley and Woodside: "Greetings, this site has been hacked. Embarcadero Media Group (Alamanac) (sic) has failed to remove content that has been harmful to the wellbeing and safety of others. Failure to honor all requests to remove content will lead to the permanent shutdown of all Embarcadero Media websites."

As part of the hack, each website's URL was replaced with the text "Unbalanced journalism for profit at the cost of human right. Brought to you by the Almanac." The message concluded with a partial tagline used by Anonymous: "We do not forgive, we do not forget, we are legion."

However, according to Forbes Magazine journalist Parmy Olson, who has written extensively about the group, one of the collective's rules is not attacking the media. In addition, no specific objection to an article or posted information was provided by the hacker.

The FBI and officers from the Palo Alto and Mountain View police departments are investigating the attack, Johnson said.

He called the incident a "sophisticated and elaborate attack aimed at inflicting extensive damage."

"Fortunately, numerous back-ups and safeguards, and a quick response by our IT team, were able to prevent even more serious harm to the company's operations," he said.

While the websites were down, the newspaper staffs published on Facebook, Twitter and Issuu.com. All the websites were restored from back-ups that are routinely made, and no content is expected to have been lost.

Embarcadero Media is headquartered at 450 Cambridge Ave., Palo Alto. The private, independent media company publishes The Almanac, which covers Menlo Park, Atherton, Woodside and Portola Valley; the Mountain View Voice, which covers the city of Mountain View; the Palo Alto Weekly, which covers Palo Alto and East Palo Alto; and the Pleasanton Weekly, which covers Pleasanton. In addition to weekly print editions, it also operates websites, including DanvilleSanRamon.com, and produces daily emailed news editions, Express. The PaloAlto app for iPhone and Android phones can be accessed through iTunes and the Google Play Store.