Former IRS Commissioner Douglas Shulman, left, and Lois Lerner, right, head of the IRS unit that decides whether to grant tax-exempt status to groups, are sworn in on Capitol Hill in Washington, Wednesday, May 22, 2013, prior to testifying before the House Oversight and Government Reform Committee hearing to investigate the extra scrutiny the Internal Revenue Service gave Tea Party and other conservative groups that applied for tax-exempt status. Lerner invoked her constitutional right to not answer questions and was dismissed.  (AP Photo/J. Scott Applewhite)

TIMELINE: Who else took the Fifth?

The IRS official enmeshed in a probe of the tax agency is the latest on a roll call of witnesses called before Congress who refused to answe…