If you’ve been monitoring the scandal over the IRS targeting of conservative groups, you’ve probably heard a lot lately about lost emails from ex-official Lois Lerner.

It’s a story that got real complicated, real fast. The following is a guide to help break it down:

What’s the problem? 

Several congressional committees continue to investigate the agency’s targeting of conservative groups, and have focused intensely on Lerner, the former head of the Exempt Organizations unit.

This is the unit that screened groups seeking a tax-exempt status known as 501(c)(4). Other IRS officials involved in that effort have cooperated with Congress, but Lerner has twice refused to testify, so lawmakers are trying to round up as many of her emails as possible to document that period.

However, Congress recently was informed that due to an apparent hard drive crash, some emails from 2009 to 2011 are gone.

Why is Congress finding out about it now? 

It’s not completely clear why this information was not disclosed earlier, but Congress was first notified in a June 13 letter – from an IRS official to the leaders of the Senate Finance Committee. After new IRS Commissioner John Koskinen pledged earlier this year to turn over all of Lerner’s email, the agency says it discovered that her files contained “very few emails” before April 2011. As it turns out, the agency says, her computer crashed in mid-2011 and some of the files are not recoverable.

So the computer crashed – but why are the emails gone? 

The IRS, like many agencies, has a rather confusing system for retaining email records. Each employee’s email limit is 500 megabytes, or roughly 6,000 emails. It was even lower prior to mid-2011. When employees hit their limit, they are supposed to move anything that qualifies as “official records” onto their hard drive (and even print their records). If a hard drive crashes, those files can be lost. That’s what the IRS claims happened here.

Isn’t there a backup system? 

Yes. The IRS says they have a system that backs up data for six months. But Koskinen testified that it’s “very costly and difficult” to extract individual emails from that system – he acknowledged, though, he didn’t know why that step was not taken here.

According to the IRS, those tapes “no longer exist” because they were recycled.

Did the IRS break the law? 

The head of the National Archives, David Ferriero, testified that the IRS “did not follow the law,” by failing to notify the Archives when the files vanished in 2011.

However, whether the IRS was violating any law in its storage of the records is a trickier question. Under IRS policy, as noted, employees are supposed to keep “official records” – on their hard drives, and in print-out form. This practice, though, has come under criticism given the huge volume of email being produced by federal agencies, and the National Archives is encouraging agencies to move to a more digital, streamlined system.