WASHINGTON—The federal agency charged with ensuring the safety of IRS employees said it has seen an uptick in the past several years in threats against agency personnel. In the past four years, there appears to have been a "steady, upward trend" in the number of threats against IRS employees, said an official with the Treasury Department's Inspector General for Tax Administration. That assessment, offered in response to an inquiry from Dow Jones Newswires, is based on preliminary data, the official cautioned. On Thursday, 53-year old Andrew Joseph Stack crashed his private plane into an Austin, Texas, office building that housed...