On Friday, the Internal Revenue Service informed Congressional investigators that it could not recover two years of emails from Lois Lerner, the former head of the agency's tax-exempt status department. Lerner has been at the center of the investigation into how and why the IRS applied additional scrutiny to the tax-exempt applications of Tea Party-affiliated organizations.

Lerner has refused to testify on the subject, meaning that her emails are one of the only records of what happened in her own words. The IRS has 67,000 emails from or to Lerner that it has or will turn over to investigators, but a large number, from 2009 to 2011, are apparently lost -- a disappearance that quickly triggered skepticism, particularly from Congressional Republicans trying to figure out whether Lerner was acting on orders from Washington.

May 22, 2013 file photo of Lois Lerner. (AP Photo/J. Scott Applewhite, File)

Here's what the IRS says happened. (A quick background on email before we begin. Email moves back and forth between servers. Email sent to IRS.gov goes to the IRS' email server; emails sent from IRS.gov to, say, gmail.com, travel over the internet to Google's email servers. You access your email using an email client, a tool that reads email from the server either directly or by downloading it first. As with everything tech-related, it is actually more complicated than this.)

Prior to the eruption of the IRS controversy last spring, the IRS had a policy of backing up the data on its email server (which runs Microsoft Outlook) every day. It kept a backup of the records for six months on digital tape, according to a letter sent from the IRS to Sens. Ron Wyden (D-Ore.) and Orrin Hatch (R-Utah). After six months, the IRS would reuse those tapes for newer backups. So when Congressional committees began requesting emails from the agency, its records only went back to late 2012.

The IRS also had two other policies that complicated things. The first was a limit on how big its employees' email inboxes could be. At the IRS, employees could keep 500 megabytes of data on the email server. If the mailbox got too big, email would need to be deleted or moved to a local folder on the user's computer.

Emails considered an "official record" of the IRS couldn't be deleted and, in fact, needed to also have a hard copy filed. Those emails that constitute an official record are ones that are loosely defined under IRS policy as ones that were "[c]reated or received in the transaction of agency business," "appropriate for preservation as evidence of the government's function or activities," or "valuable because of the information they contain". The letter sent to the senators suggests that it was up to the user to determine what emails met those standards. It's not clear if Lerner had any hard copies of important emails.

The effect of the size limit and the need to preserve records is that IRS employees have local copies of emails, particularly important ones. That's the good news. By searching Lerner's computer and those of other employees, the agency was able to compile thousands of emails sent from and to Lerner from 2011 to 2013.

And then the bad news: In 2011, Lerner's computer crashed. She requested that the IRS' information technology division try and recover the data from her hard drive. It was unable to do so, and it appears that individual machines like hers weren't backed up. (The full email chain is at the bottom of this post.)

The IRS was able to recover 24,000 emails that related to Lerner by searching other peoples' email systems and including an "earlier 2011 data collection of Ms. Lerner's email." But the whole set, it seems, is gone. (We asked the IRS for more information about the "earlier 2011 data collection," and will update this post when available.)

At least one change has arrived at the IRS in the wake of the tax-exempt status controversy: The agency now stores its backup tapes for longer than six months.