The U.S. Internal Revenue Service has been under intense scrutiny since May 10, when the tax-collection agency acknowledged that workers had used partisan criteria to intensely scrutinize conservative groups' applications for tax-exempt status.
After the release of a Treasury Department watchdog report and three congressional hearings, it is still unclear who was responsible for creating what the Treasury audit called "inappropriate" criteria for scrutinizing groups.
Lawmakers have accused top IRS officials of lying to Congress and not being more forthcoming about what they knew about the targeting.
Here is what is publicly known about the IRS scandal, based on the Treasury audit, congressional investigators and IRS officials' recent testimony:
March 2010 - A group of workers in a Cincinnati, Ohio, IRS field office begins developing partisan criteria for reviewing groups' applications for tax-exempt status. Groups whose names included the words "Tea Party" and "Patriots" were among those flagged for extra scrutiny.
The criteria were developed as that office was coping with a flood of applications after the Supreme Court's 2010 Citizens United decision that empowered the creation of 501(c)(4)s, tax-exempt groups that do not have to disclose their donors as long as they are not "primarily" engaged in political activity.
June 29, 2011 - Lois Lerner, a Washington-based official in charge of the IRS's tax-exempt office, is alerted to the targeting. She then orders that the criteria be changed to focus more generally on political or lobbying activity.
January 2012 - Without approval from IRS executives, a team of specialists changes the criteria back to terms that focused on the specific policy positions of organizations. It is not publicly known who was on this team.
March 2012 - News reports detail complaints from conservative Tea Party groups about receiving lengthy and intrusive questionnaires from IRS officials reviewing their tax-exempt applications.
March 8, 2012 - House Oversight and Government Reform Committee staff members discuss with Treasury Inspector General staff potential problems with the IRS's efforts to increase scrutiny of certain tax-exempt applications.
March 22, 2012 - House Ways and Means Oversight Subcommittee holds a hearing on general IRS operations. Chairman Charles Boustany questions then-IRS Commissioner Douglas Shulman about reports that the IRS has been targeting Tea Party groups. Shulman testifies that "what has been happening has been the normal back-and-forth that happens with the IRS" and that "there's absolutely no targeting."
Late March 2012 - The IRS's deputy commissioner for services and enforcement, Steven Miller, asks a senior technical adviser, Nancy Marks, to examine whether there are any problems in the review of tax-exempt applications for politically active groups and to make recommendations.
May 2012 - The IRS internal review concludes, and Miller and Shulman are briefed, that workers in the Cincinnati office had been using partisan criteria to review applications. Two employees are disciplined, but Miller later says it appears one of them was actually not responsible. Those individuals have not been publicly identified.
May 2012 - The criteria are again changed to become more neutral as directed by Holly Paz, a director in the Washington tax-exempt office.`
Spring 2012 - Treasury Inspector General for Tax Administration J. Russell George tells Deputy Treasury Secretary Neal Wolin that George's office had begun to review reports that the IRS was not being consistent in identifying and reviewing applications for tax-exempt status from groups involved in political advocacy.
June 28, 2012 - House Oversight Chairman Darrell Issa sends a letter to George, saying his panel understands that an inspector general's review has begun and asks for updates.
Fall 2012 - The inspector general's office lists on its website information that a review of alleged targeting by the IRS is occurring.
Throughout 2012 - Lerner gives several statements to congressional staff members saying that no targeting was occurring and that the follow-up questionnaires to conservative groups were not unusual.
April 2013 - White House counsel Kathryn Ruemmler is notified about the existence of the inspector general's review on April 16. On April 24 she is told about the probe's preliminary findings and later informs Chief of Staff Denis McDonough and other senior White House staff on what she had heard. Ruemmler decides not to brief President Barack Obama; White House spokesman Jay Carney said later that this was done to avoid any appearance of presidential interference in the review.
May 10, 2013 - Lerner makes the IRS's first public admission of the targeting, apologizing for the activity after the agency planted a question with an audience member at an American Bar Association conference. White House and Treasury senior staff members were briefed beforehand about the strategy for making the news public. Obama has said he learned of the targeting from media reports about Lerner's comments.
May 14 - George releases his office's report, which says that ineffective management led to the IRS using "inappropriate criteria" to target Tea Party and other conservative groups for extra scrutiny. The report says investigators could not determine who specifically was involved. The Justice Department opens a criminal probe into the matter.
May 17-22 - Three congressional panels hold hearings on the IRS scandal, during which Shulman and Miller say they cannot give lawmakers the names of those responsible for creating the criteria, or who was disciplined after the internal probe into the targeting. In a hearing before the House Oversight panel, Lerner says she did nothing wrong and then declines to answer questions, citing her constitutional right to avoid self-incrimination.
(Reporting By Karey Van Hall with additional reporting by Patrick Temple-West and Kim Dixon; Editing by Tim Dobbyn)