Two top House Republicans on Friday called for Attorney General Eric Holder to appoint a special prosecutor to investigate the alleged targeting of conservative groups by the Internal Revenue Service.

House Ways and Means Committee Chairman Dave Camp, R-Mich., and House Judiciary Committee Chairman Bob Goodlatte, R-Va., cited a missing batch of IRS emails as part of the reason Holder should appoint an independent investigator to look into the IRS targeting.

They also pointed to new emails showing former IRS officials Lois Lerner sought an audit of Sen. Chuck Grassley, R-Iowa, over an invitation to a seminar that was mistakenly sent her way.

“It is clear that appointing an outside Special Counsel to investigate this matter would be in the public interest,” Camp and Goodlatte said in the letter to Holder. “ The American people are very concerned that their government has targeted individual American citizens for harassment solely on the basis of their political beliefs. They deserve to know who ordered the targeting, when the targeting was ordered, and why.”

It's been a year since a Treasury Department investigation revealed evidence that Lerner's tax exempt division had improperly singled out conservative and Tea Party groups seeking tax exemption.

Congressional Republicans say they have yet to receive thousands of emails and other documentation related to the matter.

Earlier this month, the IRS disclosed that it had lost two years of Lerner’s emails due to a hard drive crash. This week, the House Ways and Means panel disclosed an email from Lerner suggesting an IRS audit of Grassley, who during his tenure as the top Republican on the Senate Finance Committee had provided rigorous oversight of Lerner’s division.

House Republicans in May passed a resolution calling on Holder to appoint a special counsel to look into the targeting, but Holder has essentially ignored it. Camp and Goodlatte believe new developments now compel Holder to act.

The lost emails and Grassley targeting, Camp and Goodlatte said in the letter, “have served to wash away whatever credibility the administration retained in this matter and have indisputably created the ‘extraordinary circumstances’ called for in the regulations.”