Police Chief John M. Kelly told the Boston Herald the publication of personal information was a "human error."
"We've been doing (the online log) for eight years. This is the first time anything like this has happened," he said.
Normally, the police department posts the names, dates of births, addresses and charges of those arrested in the log, but never Social Security numbers, Kelly said. The blunder resulted in the posting of interactions of 110 people who contacted the police between Jan. 7 and Jan. 13, including those involved in minor traffic stops and medical calls -- interactions that are not normally posted online, the Herald reported.
Kelly said the mistake happened when an employee failed to check the appropriate box that would automatically block the Social Security numbers from being posted online.
The information stayed online for 10 days before it was taken down. Police called or wrote letters to all 110 individuals, offering an apology and a free identity theft kit., the Herald reported.
"I extend my sincere apologies to you for the discomfort this error has caused and I will do everything I can to assist those adversely affected by it," Kelly wrote in a letter.
He said the department had reviewed and altered its procedures to prevent a similar incident from happening in the future.
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