Sen. Marco Rubio (R-Fla.) on Monday called for the resignation of the Internal Revenue Service commissioner after the agency admitted targeting conservative groups seeking tax-exempt status.

“It is clear the IRS cannot operate with even a shred of the American people’s confidence under the current leadership,” Rubio wrote in a letter to Treasury Secretary Jack Lew. “Therefore, I strongly urge that you and President Obama demand the IRS Commissioner’s resignation, effectively immediately.

HillTube video: Congress to investigate IRS targeting

“No government agency that has behaved in such a manner can possibly instill any faith and respect from the American public,” Rubio added.

The IRS on Friday apologized after disclosing that some conservative groups, including ones with the words “Tea Party” or “patriot” in their names, were targeted for higher scrutiny.

{mosads}In his letter, Rubio, however said “such behavior cannot be excused with a simple apology.”

The actions targeting Tea Party groups occurred under the watch of former Commissioner Doug Shulman, whose term ended in 2012.

The current acting commissioner is Steven Miller, who previously served as deputy commissioner for Services and Enforcement.

Rubio spokesman Alex Conant said Rubio’s letter referred to Miller. “He was deputy commissioner when this occurred,” Conant said in an email.

The IRS has said that low-level employees in the organization’s Cincinnati, Ohio, office were responsible and denies that the actions were politically motivated.

Lawmakers, though, have expressed anger and vowed to probe the matter. A report from a federal watchdog leaked over the weekend revealed that despite public denials from the IRS, top officials were aware of the targeting scandal for more than two years.

A number of conservative organizations have also criticized the IRS over the issue. On Monday, Jenny Beth Martin, the co-founder of Tea Party Patriots, the largest Tea Party organization, said an apology was not enough.

“Congress needs to investigate this and find out how many more lies the IRS is telling,” Martin said.

Rubio, a Tea Party favorite, is seen as a potentially strong 2016 presidential contender.

Read the full text of Rubio’s letter below:

The Honorable Jack Lew

Secretary

U.S. Department of the Treasury

1500 Pennsylvania Avenue, NW

Washington, D.C. 20220

Dear Secretary Lew:

Recent revelations about the Internal Revenue Service’s selective and deliberate targeting of conservative organizations are outrageous and seriously concerning. This years-long abuse of government power is an assault on the free speech rights of all Americans. This direct assault on our Constitution further justifies the American people’s distrust in government and its ability to properly implement our laws.

The American people deserve answers about how such seemingly unconstitutional and potentially criminal behavior could occur, and who else was aware of it throughout the Administration. It is imperative that you, your predecessor, and other past and present high-ranking officials at the Department of Treasury and IRS immediately testify before Congress.

The public expects your complete cooperation with both congressional investigations and potential criminal inquiries. If investigations reveal that bureaucrats or political appointees engaged in unconstitutional or criminal targeting of conservative taxpayers, they must be prosecuted to the fullest extent of the law. At a bare minimum, those involved with this deeply offensive use of government power have committed a violation of the public trust that has already had a profoundly chilling effect on free speech. Such behavior cannot be excused with a simple apology.

Furthermore, it is clear the IRS cannot operate with even a shred of the American people’s confidence under the current leadership. Therefore, I strongly urge that you and President Obama demand the IRS Commissioner’s resignation, effectively immediately. No government agency that has behaved in such a manner can possibly instill any faith and respect from the American public.

Sincerely,

Marco Rubio

This story was last updated at 11:13 a.m.