During a joint press conference with Turkish Prime Minister Recep Tayyip Erdogan, President Obama called for an investigation into the IRS, saying, “We’ve got to make sure that it is doing its job scrupulously.” (Sandi Moynihan/The Washington Post)

Senior lawmakers investigating what went wrong at the Internal Revenue Service are planning to focus on whether IRS officials misled Congress about a policy that targeted conservative groups for extra screening when seeking a tax exemption, congressional aides say.

With the first high-profile hearing on the topic set to begin Friday morning, congressional aides say lawmakers in both chambers will seek answers about why they weren’t told that the IRS had singled out conservative groups for scrutiny despite multiple inquiries in recent years.

“I’m not an unreasonable guy, but hiding what happened to Congress is inexcusable and how this was allowed to happen must be answered,” Sen. Orrin G. Hatch (Utah), the top Republican on the Senate Finance Committee, said through a spokeswoman Thursday.

The questions from lawmakers come amid a rising furor over the IRS’s unfair treatment of conservative groups during an 18-month period that ended in early 2012, spawning FBI and Justice Department criminal investigations in addition to at least four congressional probes.

This week, President Obama demanded the resignation of acting IRS commissioner Steven T. Miller, replacing him Thursday with budget official Daniel Werfel, a veteran of Republican and Democratic administrations. Also on Thursday, Joseph Grant, commissioner of the agency’s tax exempt/government entities divisions, announced that he would soon retire.

The Friday hearing before the House Ways and Means Committee, which oversees the IRS, will be the first of several sessions in coming weeks where lawmakers will grill current and former officials about the IRS’s screening practices.

Tea party organizations and other conservative groups seeking tax-exempt status increasingly complained in recent years that they were receiving overly detailed questionnaires from the IRS and facing long delays.

Republican officials wrote letters and held public and private briefings with top IRS officials demanding to know whether the IRS was giving extra scrutiny to conservative groups. Each time, agency officials said no.

But a report by the Treasury inspector general for tax administration said senior agency officials became aware of what was happening more than a year ago, raising questions about whether IRS officials had been forthcoming with Congress.

For example, in May 2012, according to congressional aides briefed by the IRS and inspector general, Miller learned that targeting had been occurring. But in September, he sent a response to GOP senators asking questions that did not acknowledge the screening.

Lawmakers now say those statements and others from IRS officials will be a core focus of the inquiry, with multiple congressional committees investigating the matter. Miller is scheduled to testify Friday along with Inspector General J. Russell George, who conducted the audit on screening practices released this week.

What the IRS told Congress could also be part of criminal inquiries. In testimony on Wednesday, Attorney General Eric H. Holder Jr. said one possibility that investigators will explore is whether “false statements” were made in the IRS case.

The House Oversight and Government Affairs Committee and Senate Finance Committee also plan their own hearings next week. The House oversight panel is focusing on disclosures by Lois Lerner, the direct supervisor of the tax-exempt unit.

In a letter to Lerner this week, Rep. Darrell Issa (R-Calif.), the committee’s chairman, wrote: “We are concerned that information you have provided to the committee related to this matter on prior occasions was false or misleading.”

IRS officials did not respond to a request for comment late Thursday.

Ed O’Keefe contributed to this report.

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