From the Spokesman-Review: "Murals depicting white settlers accosting and then hanging a shirtless Native American man have been displayed openly in Idaho’s temporary Statehouse for the past year, with a note from the state Historical Society promising that interpretation of the murals is 'forthcoming.'" The controversy of the mural goes back at least to 2006. The mural pictured here is apparently one is a series.
The murals were painted in the 1930s as part of a WPA project by artist Martin Fletcher. But Fletcher abandoned the project when he discovered that he could make more money painting portraits. The paintings were finished by "no fewer than 25 relief workers." When Martin toured the finished paintings he said they were "acceptable, but certainly nothing to write home about." Martin noted that "The finished product was further impaired when companion panels somehow were separated and now appear on different floors. For example, Indians charging down one staircase wall menace a steam locomotive, while the covered wagon train the Indians were suppose to be attacking hangs safely on another floor."
Interestingly, Martin's original design did include captions to explain the history that they were meant to depict--though I cannot find any record of what those captions were supposed to say. The murals were controversial from the time of their unveiling and lynching mural has spent much of the last decades hidden under a flag. The plaques that are going up (Plaque 1 | Plaque 2) have a generic quality to them, as one might expect of any committee-written prose: "To the California-based artists of the Great Depression era who completed this work, the murals represented their concept of how the West was won. During the 1930s, government-sponsored painters sought to portray strength and triumph, a nation overcoming adversity to settle a vast land."
This interpretation seems off the mark to me. The mural here is clearly not a celebration. The Indian looks too innocent and helpless, the white lynchers too sinister and matter-of-fact. People in the 1930s, when lynchings of African Americans and immigrants were still often in the news, would have recognized the mural as a crude metaphor for America's unjust treatment of American Indians.
Most likely Fletcher intended a piece of social commentary similar to some of Thomas Hart Benton's contemporaneous murals. See for example this panel from Benton's series "American Historical Epic" at the Nelson-Atkins Museum in Kansas City. Fletcher's vision was lost when the actual painting of the murals fell to untrained WPA workers.
By the way, here is a great site that attempts to index all of the surviving WPA public art.