WASHINGTON -- Moving to quell a growing scandal, President Barack Obama on Wednesday fired the acting chief of the Internal Revenue Service and vowed to work closely with Congress in determining who ordered lower-level employees to target tea party groups and other conservative organizations.

“It’s inexcusable and Americans are right to be angry about it,” Obama said at the White House after meeting with top Treasury Department officials. “And I’m angry about it.”

He said Treasury Secretary Jacob Lew asked for and accepted the resignation of acting IRS Commissioner Steven Miller, the first high level casualty of the scandal that’s erupted since a Treasury Department’s inspector general confirmed what he called the inappropriate scrutiny of conservative groups applying for tax-exempt status.

In an internal memo to IRS employees, Miller said his assignment as acting chief would end in June, suggesting he may still appear at hearings scheduled by the House Ways and Means Committee on Friday and the Senate Finance Committee on Tuesday.

“This has been an incredibly difficult time for the IRS given the events of the past few days, and there is a strong and immediate need to restore public trust in the nation’s tax agency,” Miller wrote to his colleagues. “I believe the service will benefit from having a new acting commissioner in place during this challenging period. As I wrap up my time at the IRS, I will be focused on an orderly transition.”

A 25-year veteran of the tax agency, Miller had headed the IRS as acting director since November. Most of the alleged wrongdoing occurred well before Miller took over as acting chief, when the agency was run by Commissioner Doug Shulman, who was appointed by President George W. Bush. Obama did not say who might take the helm of the embattled agency.

Republicans welcomed Obama’s announcement that Miller had been fired.

“A great number of questions remain; they demand answers. The resignation of Steven Miller is a positive and important step as this agency struggles to try to regain the public’s trust,” said Sen. Orrin Hatch of Utah, the top Republican on the Senate Finance Committee. “A clean slate at the IRS with new leadership is imperative to fix this egregious encroachment on the lives of honest, hard-working Americans whose only sin was that they want to express their beliefs.”

Congress will seek more answers about how the scandal occurred, and who knew about it.

The administration has said it first learned of the pending special inspector general’s report around April 22, when a Treasury official told the White House counsel’s office. But Obama said he learned about it Friday when the American public did. The White House has not said why such an important matter was not shared with the president weeks ago, or whether Lew and others were made aware of it then but did not tell the president.

Some Republican lawmakers have complained about unfair treatment of tea party groups by the IRS for two years, and they suggested that the tax treatment effectively suppressed the vote in 2012 election.

“If the president is as concerned about this issue as he claims, he’ll work openly and transparently with Congress to get to the bottom of the scandal – no stonewalling, no half-answers, no withholding of witnesses,” said Senate Minority Leader Mitch McConnell, R-Ky.