At 10 p.m on May 14, 2004, during an arts festival in northeast Minneapolis, a 24-year-old named Jackson Mahaffy was crossing the street when he bumped into a dark, slow-moving SUV with two off-duty police sergeants inside.
Robert Kroll and the second sergeant hopped out and began hitting Mr. Mahaffy, according to interviews and court documents. Not realizing the two men in civilian clothes were police officers, Mr. Mahaffy’s sister and at least three others came to his defense. The officers attacked them too; Mr. Kroll kicked…