Reps. Cheri Bustos, D-Ill., and Dave Loebsack, D-Iowa, called for independent investigations of the Internal Revenue Service for singling out organizations for extra scrutiny based on their political leanings.

Bustos called it "deeply troubling" and condemned "this apparent abuse of power in the strongest possible terms." She said on Tuesday that those found responsible should be held accountable.

Loebsack, meanwhile, said firings should be considered.

"If it is found that these allegations are true, I urge the swift dismissal of those involved in the wrongdoing" as well as anyone who knew about it and didn't act to investigate, he said.

The IRS acknowledged last week that it singled out conservative groups that had applied for tax-exempt status. On Tuesday, the Justice Department said a criminal investigation has been opened.

The IRS apologized last week and said the extra scrutiny was the work of lower-level employees in its Cincinnati office. However, The Associated Press reported Tuesday that offices in California and Washington, D.C., also sought "extensive information" from tea party groups asking for nonprofit status.

The AP also reported that Sen. Chuck Grassley, R-Iowa, sent a letter to the IRS concerning the timing of its disclosure that it had targeted tea party and other conservative groups.

Grassley asked for records relating to the agency's decision to make the disclosure at a conference last Friday rather than to Congress, which has been asking for some time about the allegations.

Tea party groups complained during the 2010 election cycle about extra scrutiny.

Grassley also asked to know about any contacts with the White House on the matter.

Colin Milligan, a spokesman for Bustos, said the House and Senate committees with oversight of the IRS ought to appoint an independent commission, "made up of people not in Congress or the Administration" to investigate the matter.

He said Bustos thinks such an independent investigation is needed to avoid "political gamesmanship that all too often occurs inside congressional hearing rooms and for our government to make real progress towards regaining the public trust."