Treasury Department Inspector General for Tax Administration J. Russell George turns out to be a bulldog type of watchdog. And a bulldog attitude is apparently needed to get to the bottom of seemingly rampant wrongdoing at the Internal Revenue Service.

Mr. George’s office (TIGTA) told Congress Thursday that it is investigating “potential criminal activity” by the Internal Revenue Service in providing false information to its own investigation, a federal court and Congress about the availability of emails related to its targeting of non-profit groups promoting conservative politics.

Back in August 2013 the House of Representatives issued a subpoena to the IRS for the emails of Lois Lerner, director of the agency’s exempt organizations unit.

Her office led the crack-down on conservative non-profits from 2010 through 2012. Ms. Lerner was later cited for contempt of Congress because she refused to testify about her activities at the IRS.

And in June 2014, after a delay of 10 months, the IRS told Congress it could not recover Lerner’s emails to the White House, the Justice Department, the Federal Election Commission, the Treasury Department, Democratic members of Congress and other agencies outside the IRS because her computer with the relevant emails had crashed and no backups were available.

IRS Commissioner John Koskinen testified that Ms. Lerner’s laptop hard drive had been “recycled and destroyed in the normal process,” and that backup tapes were routinely overwritten every six months.

Other IRS officials provided similar testimony to a federal court.

However, Inspector General George’s separate investigation last year found the backups that the IRS claimed had routinely been erased. On Thursday, TIGTA Deputy IG for Investigations Tim Camus told the House Oversight Committee that it took his agents two weeks to find the missing tapes and another four months of work by technicians to assemble Lerner’s emails from the tapes. “They were right where you would expect them to be,” he said.

Mr. Camus said no one from Mr. Koskinen’s office or the top level of the IRS had ever asked the agency’s technology specialists for the tapes.

He added that his agents had discovered an additional 400 backup tapes that the IRS did not initially disclose to his investigation, that hard drives said by the IRS to have been destroyed actually exist — and that “some tapes” may have been erased.

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

It is too soon to say whether these facts reveal a criminal conspiracy or simply monumental incompetence at the IRS.

But these troubling matters certainly bear further investigation.