IRS Commissioner John Koskinen is headed back to Capitol Hill to explain why his agency failed to search 760 exchange server drives that may contain Lois Lerner's emails.

"The committee has recently learned that the Treasury Inspector General for Tax Administration has identified 760 exchange server drives from the relevant period that currently exist and could contain Ms. Lerner's emails," said Rep. Jim Jordan, R-Ohio, in a letter to Koskinen made public Friday.

"According to TIGTA, the IRS did not search these sources for Ms. Lerner's emails during its process of producing documents to Congress because the IRS was not aware that the exchange servers even existed," Jordan said.

Jordan is chairman of a key subcommittee of the House Oversight and Government Reform Committee, which for more than a year has been probing the tax agency's illegal targeting and harassment of Tea Party and conservative nonprofit applicants during the 2010 and 2012 campaigns.

Lerner is the former director of the IRS tax-exempt organizations division in which the targeting was conducted.

Koskinen has repeatedly claimed that Lerner's computer hard drive crashed, making it impossible to recover her emails during the targeting period.

In addition to requests from multiple congressional committees for Lerner's emails, the IRS is under order by a federal judge hearing a civil suit against the tax agency to explain in sworn detail what it did to recover the communications.

Koskinen will appear before Jordan's subcommittee Sept. 17.

Mark Tapscott is executive editor of the Washington Examiner.