The IRS’s inspector general confirmed Thursday it is conducting a criminal investigation into how Lois G. Lerner’s emails disappeared, saying it took only two weeks for investigators to find hundreds of tapes the agency’s chief had told Congress were irretrievably destroyed.

Investigators have already scoured 744 backup tapes and gleaned 32,774 unique emails, but just two weeks ago they found an additional 424 tapes that could contain even more Lerner emails, Deputy Inspector General Timothy P. Camus told the House Oversight Committee in a rare late-night hearing meant to look into the status of the investigation.

“There is potential criminal activity,” Mr. Camus said.

He said they have also discovered the hard drives from the IRS’s email servers, but said because the drives are out of synch it’s not clear whether they will be able to recover anything from them.

“To date we have found 32,744 unique…