Sen. Carl Levin, D-Mich., repeatedly pressed the Internal Revenue Service to investigate the tax-exempt status of specific conservative nonprofit organizations in letters to then-IRS commissioner Doug Shulman and director Lois Lerner in 2012.

Levin said he was concerned nonprofit organizations were abusing their tax-exempt status and engaging in partisan politics and requested information from the IRS on 12 organizations.

"Organizations are using Internal Revenue Code section 501(c)(4) to gain tax exempt status while engaging in partisan political campaigns," wrote Levin in one letter on July 27, 2012. "Making the problem worse is that the IRS knows there is a problem because of the public nature of the activity but has failed to address it."

He asked whether the 12 organizations "applied for [tax-exempt status]; and if so ... received the described exemption for political activity from the IRS."

Levin's list contained nine conservative groups, including Club for Growth, Americans for Tax Reform, and Americans for Prosperity. It also included two liberal groups and one centrist group.

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The IRS officials were reportedly already aware that the agency had been targeting conservative groups for special scrutiny during the time Levin was corresponding with Shulman and Lerner.

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