The IRS mistakenly posted the Social Security numbers of tens of thousands of Americans on a government website, the agency confirmed Monday night. One estimate put the figure as high as 100,000 names.

The numbers were posted to an IRS database for tax-exempt political groups known as 527s and first discovered by the group Public.Resource.org.

The California-based group said it learned of the “privacy breach” Tuesday while working on an unrelated audit of an “improperly vetted shipment” of IRS data on DVDs and promptly informed the agency, which shut down the site the next day.

An IRS spokesman told FoxNews.com on Monday the agency was alerted about a “substantial number” of Social Security numbers posted on the site and removed web access to the information “out of an abundance of caution.” The spokesman also said the IRS is now “assessing the situation and exploring available options.”

A message on the agency’s 527 homepage asks visitors to check back Monday, but the site was still down Monday evening.

Read more at Fox News.