The 15 people President Trump pardoned Tuesday evening include the first two congressmen who endorsed him for president — former Reps. Chris Collins (R-N.Y.) and Duncan Hunter (R-Calif.), both convicted of financial crimes — two people jailed in Special Counsel Robert Mueller's Russia investigation, and four private guards working for Blackwater who were serving long sentences for an unprovoked and unnecessary 2007 massacre of civilians in Baghdad's Nisour Square that left 17 Iraqis dead, including two boys, ages 8 and 11.

Blackwater, since sold and renamed Academi, is a private military contractor outfit headed at the time by Erik Prince, brother to Trump's education secretary, Betsy DeVos. The Nisour Square massacre marked a low point in U.S.-Iraqi relations after the 2003 U.S. invasion, and federal prosecutors spent years bringing the four Blackwater guards — Nicholas Slatten, Paul Slough, Evan Liberty, and Dustin Heard — to justice.

After a federal judge in 2009 dismissed the first murder and manslaughter convictions of the Blackwater contractors, ruling the evidence was tainted, then-Vice President Joe Biden said at a press conference in Baghdad that the men had not been acquitted and the U.S. would appeal the decision. "In subsequent years, as the case continued, the contractors became known in conservative media as the 'Biden Four,'" The Washington Post reports. Slatten was eventually sentenced to life in prison for first-degree murder, while the other three were convicted of manslaughter and given sentences of 12 to 15 years.

"Campaigns urging that the four receive presidential pardons began in earnest last year, most arguing that the men were veterans still in engaged in quasi military duties," the Post reports, noting that Trump has already pardoned two Army officers convicted or awaiting trial on murder charges for shooting Afghan civilians. In a 2009 column at Fox News, Duncan Hunter — still in Congress — called "the Biden Four" brave "political pawns" who were "sent to prison for doing their jobs."/ The Week