"They had to do a massive download because they were not going to have access to the mainframes for several months," he said. That data included the Social Security numbers of over 1.3 million voters out of the 2.2 million registered voters in the city, and was "unfortunately included with the basic information" made available.

Allen added that the election board has hand-delivered letters on Monday to ward offices requesting the disks to be returned. He added that so far there is no indication that any of the information has been used for fraudulent purposes.

Since the board moved back to the Cook County building, it has not been including any Social Security numbers with the information sent out, Allen said.

The breach was brought to the Board's attention by Peter Zelchenko, who was running for an alderman's position, last month. Zelchenko is part of a group called the Illinois Ballot Integrity Project, and last October he disclosed a major breach on the Chicago Election board's Web site. That breach not only allowed users to view the Social Security numbers of registered voters but to also actually edit and delete the information.

Since that disclosure, the Board has fixed the problem and removed all but the last four digits of the social security numbers listed on its site, Allen said. It also hired a local firm, Grant Thornton LLP, to help deal with data security and storage issues as well as the control and dissemination of data.