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

The California-based group said it learned of the "privacy breach" 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. The group said the Social Security numbers were largely those of donors, though some were also from people who prepared tax returns to furnish to the IRS. 

An IRS spokesman told 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. founder Carl Malamud told on Monday night that roughly 100,000 Social Security numbers were exposed.

Malamud said in a statement on the group's website that it hopes the Obama administration will act to restore access to the agency's nonprofit database and resolve its concerns over what it described as a "serious violation of federal law."

"It is time now for the administration to send a tiger team over to the [IRS] to help fix their information management practices," Malamud said.