Treasury Inspector General for Tax Administration Russell George and his deputy, Timothy Camus, will tell the House Oversight Committee on Thursday that their investigation was able to recover more than 1,000 emails
the IRS did not turn over to Congress, according to the written testimony.
Lerner was the Internal Revenue Service official at the center of allegations that the agency targeted tea party groups applying for nonprofit status. Congress requested Lerner's emails from the IRS and agency officials told lawmakers an unknown number of emails had been lost when Lerner's computer crashed.
The investigation could not determine why Lerner's computer crashed, according to the testimony.
"Prior to our investigation and our efforts to recover Ms. Lerner's missing e-mails; the IRS did not search for, review or examine the two separate sources of backup tapes, the server hard drives, or the loaner laptops that ultimately produced new, previously undisclosed e-mails," according to the testimony.
Thursday's hearing will focus in part on why the IRS did not search through those additional sources, a senior committee aide said.
Lawmakers will also work to establish a clear timeline of events starting when President Barack Obama said in February 2014 that the IRS scandal did not have "even a smidgen of corruption."
The committee wants to know what IRS Commissioner John Koskinen knew and when, the aide said.
The inspector general's testimony said the more than 400 backup tapes most likely to have contained the missing emails were erased in March 2014 -- a month after the IRS realized it was missing some of Lerner's emails.
But the independent investigation "did not uncover evidence that the erasure was done ... to destroy evidence or conceal information from Congress," according to the written testimony.
The inspector general is wrapping up the investigation and noted there's a "remote possibility" that they may find additional emails, according to the testimony.
In a statement Thursday, the IRS said it has "fully cooperated with the investigation," but acknowledged the backup tapes should not have been erased.
"The IRS recognizes there was a clear breakdown of communication in one part of the organization regarding the need to preserve and retain the back-up tapes and information," the IRS said, noting the review found the erasure wasn't intentional.