Shulman was appointed by President George W. Bush. His 6-year term ended in November. President Barack Obama has yet to nominate a successor. The agency is now being run by acting Commissioner Steven Miller.
Rep. Charles Boustany, R-La., chairman of the Ways and Means oversight subcommittee, requested a trove of documents from the IRS on Friday, including all communications containing the words "tea party" and "patriot."
House Ways and Means Committee Chairman Dave Camp, R-Mich., said Friday he will hold a hearing on the matter has not yet set a date.
"The IRS absolutely must be non-partisan in its enforcement of our tax laws," Camp said. "We will hold the IRS accountable for its actions."
House Majority Leader Eric Cantor, R-Va., and the House Oversight and Government Reform Committee have also promised investigations.
Treasury spokesman Anthony Coley said the department will support the inspector general's investigation.
"The Treasury Department expects all individuals and organizations to be treated fairly by the IRS. Anything less is inappropriate and unacceptable."
There has been a surge of politically active groups claiming tax-exempt status in recent elections — conservative and liberal. Among the highest profile are Republican Karl Rove's group, Crossroads GPS, and the liberal Moveon.org.
These groups claim tax-exempt status under section 501 (c) (4) of the federal tax code, which is for social welfare groups. Unlike other charitable groups, these organizations are allowed to participate in political activities but their primary activity must be social welfare.
That determination is up to the IRS.
Lerner said the number of groups filing for this tax-exempt status more than doubled from 2010 to 2012, to more than 3,400. To handle the influx, the IRS centralized its review of these applications in an office in Cincinnati.
Lerner said this was done to develop expertise among staffers and consistency in their reviews. As part of the review, staffers look for signs that groups are participating in political activity. If so, IRS agents take a closer look to make sure that politics isn't the group's primary activity.
As part of this process, agents in Cincinnati came up with a list of things to look for in an application. As part of the list, they included the words, "tea party" and "patriot," Lerner said.
"It's the line people that did it without talking to managers," Lerner told The AP. "They're IRS workers, they're revenue agents."
In all, about 300 groups were singled out for additional review, Lerner said. Of those, about a quarter were singled out because they had "tea party" or "patriot" somewhere in their applications.
The IRS statement said that once applications were chosen for review, they all "received the same, even-handed treatment."
Lerner said 150 of the cases have been closed and no group had its tax-exempt status revoked, though some withdrew their applications.
"Mistakes were made initially, but they were in no way due to any political or partisan rationale," the IRS said in a statement. "We fixed the situation last year and have made significant progress in moving the centralized cases through our system."
"I don't think there's any question we were unfairly targeted," said Tom Zawistowski, who until recently was president of the Ohio Liberty Coalition, an alliance of tea party groups in the state.
Zawistowski's group was among many conservative organizations that battled the IRS over what they saw as discriminatory treatment. The group first applied for nonprofit status in June 2009, and it was finally granted on Dec. 7, 2012, he said — one month after Election Day.
"It is suspicious that the activity of these 'low-level workers' was unknown to IRS leadership at the time it occurred," said Jenny Beth Martin, national coordinator for Tea Party Patriots, which describes itself as the nation's largest tea party organization. "President Obama must also apologize for his administration ignoring repeated complaints by these broad grass-roots organizations of harassment by the IRS in 2012, and make concrete and transparent steps today to ensure this never happens again."