Key dates regarding the missing emails sought in the investigation into the IRS' targeting of tea party groups:
June 13, 2011: Lois Lerner, the IRS official at the center of the investigation, reports her computer's hard drive has crashed, according to an email from another member of the Exempt Organizations Division, which Lerner led.
June 29, 2011: Lerner first learns that groups with "Tea Party," ''Patriot" or "9/12 Project" are being targeted for extra scrutiny by members of her staff, according to a report from Treasury inspector general for tax administration.
Aug. 5, 2011: Lerner is told that the data on her hard drive is unrecoverable, according to an email provided to Congress.
December 2011: The computer of Lerner's boss' chief of staff, Nikole Flax, crashes.
June 2012: Treasury inspector general selects cases of applications for tax-exempt status for review after concerns had been raised months earlier in the media.
May 3, 2012: Rep. Dave Camp, R-Mich., chairman of House Ways and Means Committee, sends letter to IRS commissioner requesting all applications seeking tax-exempt status in 2010 and 2011, including all files, correspondence and internal IRS records related to the applications.
May 10, 2013: Lerner apologizes on behalf of IRS for "inappropriate" targeting of conservatives. White House says the matter is already being investigated by an inspector general.
May 14, 2013: Treasury inspector general releases report finding that IRS supervisors knew since 2011 that conservative groups applying for tax exempt status were being unfairly targeted. The Justice Department says it will conduct a criminal investigation.
Late May/early June 2013: IRS asks certain employees to retain records.
February 2014: IRS realizes emails are missing.
May 8, 2014: IRS promises to turn over all Lerner's emails to the House Ways and Means Committee.
June 13, 2014: IRS tells Congress some of Lerner's emails are missing because her computer crashed in 2011.
June 20, 2014: IRS tells Congress that a total of eight employees whose emails are sought in the congressional investigation experienced computer crashes, resulting in an unknown amount of lost data.