A series of new revelations Wednesday and Thursday put the Internal Revenue Service back under fire for its alleged efforts to curtail the power of conservative nonprofits.

First, the Government Accountability Office uncovered evidence that holes in the tax agency's procedure for selecting nonprofit groups to be audited could allow bias to seep into the process.

Then, during a heated House Ways and Means Committee hearing Thursday morning, lawmakers exposed the lack of safeguards that could prevent IRS officials from going after groups with which they disagreed.

Meanwhile, the conservative watchdog Judicial Watch released documents Wednesday that suggested the IRS targeted the donors of certain tax-exempt organizations.

The controversies focused renewed scrutiny on the embattled agency, which has been fending off allegations of discrimination against conservatives since 2013.

"The burden of proof is on the IRS to show they are not targeting organizations," said Rep. Kristi Noem, R-S.D., during the hearing Thursday.

She noted "up to 34 percent of cases selected for audit were dismissed without documentation," suggesting IRS officials could have given certain groups "preferential treatment" by declining to audit them.

"How do we know that those decisions weren't biased?" Noem asked the IRS commissioner, John Koskinen.

Rep. Peter Roskam, chairman of the Ways and Means Committee's oversight subcommittee, highlighted a finding in the GAO report that indicated one in four audits that were touched off by a complaint had no record of the original allegation on file.

"This means no one can go back and determine if the audit was begun for a fair reason, or an unfair reason," Roskam said.

Koskinen dodged Roskam's attempts to prompt an admission that there were ulterior motives behind the tax agency's targeting of conservatives.

The IRS commissioner noted several active investigations had yet to identify any ideologically-driven intent behind the scrutiny of right-leaning groups.

"It's not my role to preempt those investigations and conclude one way or the other," Koskinen said.

Michelle Easton, president of the Clare Boothe Luce Policy Institute, described during the hearing the costly audit her conservative organization faced in 2011.

"I never believed the IRS selected the institute randomly," Easton said.

Joseph Metzger, vice president of finance at the Leadership Institute, said his conservative nonprofit spent $50,000 and hundreds of staff hours battling an audit that same year.

"We would've had more rights to protect ourselves against a $25 parking ticket than a $50,000 audit," Metzger said.

He noted that after his group filed a Freedom of Information Act request in an effort to uncover the reasoning behind the audit, the IRS sent back copies of the Leadership Institute's own document submissions.

The records released by Judicial Watch include an internal email from June 2011 that reveals at least five donors to an unnamed nonprofit were under IRS scrutiny.

Although the IRS had not enforced the gift tax since 1982, according to Judicial Watch, the IRS claimed it needed to begin reviewing donor lists to some tax-exempt groups for the purposes of imposing the tax on larger donations.

"These documents that we had to force out of the IRS prove that the agency used donor lists to audit supporters of organizations engaged in First Amendment-protected lawful political speech," said Tom Fitton, president of Judicial Watch.

Fitton said the documents suggested the IRS "was willing to illegally tax and audit citizens to shut down opposition to Barack Obama's policies and re-election."