Sen. Ted Cruz has called on Attorney General Eric Holder to appoint a special prosecutor to investigate the Internal Revenue Service's controversial targeting of conservative groups.

The Obama administration last year vowed to get to the bottom of the IRS's singling out groups for special scrutiny that included the conservative buzzwords "Tea Party," "patriot" or "9/12" in their names. But Cruz, a Texas Republican and vocal Obama critic, accused the administration of slow-walking its investigation and falling short of its promise to hold accountable those responsible for any wrongdoing.

"To date, no one has been indicted. And it has been reported that the FBI plans not to file any criminal charges," Cruz wrote in a letter to Holder.

Cruz, a Tea Party favorite, said the prosecutor must have "meaningful independence" to investigate potential criminal action and to "follow the facts wherever they may lead, including the White House."

Cruz also rebuked the White House for appointing Barbara Bosserman, an Obama campaign donor, to run the Justice Department's investigation of the IRS scandal. And the Texan is upset the administration bypassed Congress to implement new IRS rules that conservatives say would make it legal for the agency to unfairly target Tea Party groups, a move the lawmaker says would "stifle free speech" among nonprofit organizations.

"This is a sad state of affairs," Cruz wrote. "The widespread perception of partisan bias, of manifest conflict of interest, besmirches the reputation of the Department of Justice. And it undermines confidence in the rule of law."

Read Ted Cruz's letter to Eric Holder below: