TIMOTHY MESSER-KRUSE doesn't remember her name, but the question she asked in his college classroom a dozen years ago changed his career--and now it may revolutionize everything historians thought they knew about a hallowed event in the imagination of the American Left. "In my courses on labor history, I always devoted a full lecture to Haymarket," says Messer-Kruse, referring to what happened in Chicago on the night of May 4, 1886. He would describe how a gathering of anarchists near Haymarket Square turned into a fatal bombing and riot. Although police never arrested the bomb-thrower, they went on to tyrannize radical groups throughout the city, in a crackdown that is often called America's first Red Scare. Eight men were convicted of aiding and abetting murder. Four died at the end of a hangman's noose. Today, history books portray them as the innocent victims of a sham trial: They are labor-movement martyrs who sought modest reforms in the face of ruthless robber-baron capitalism.

As Messer-Kruse recounted this familiar tale to his students at the University of Toledo in 2001, a woman raised her hand. "Professor," she asked, "if what it says in our textbook is true, that there was 'no evidence whatsoever connecting them with the bombing,' then what did they talk about in the courtroom for six weeks?"

The question stumped Messer-Kruse. "It had not occurred to me before," he says. He muttered a few words about lousy evidence and paid witnesses. "But I didn't really know," he recalls. "I told her I'd look it up." As he checked out the standard sources, he failed to find good answers. The semester ended and the student moved on, but her question haunted him. "My interest grew into an obsession." As Messer-Kruse began to look more closely, he started to wonder if the true story of Haymarket was fundamentally different from the version he and just about everybody else had been told.

The 49-year-old Messer-Kruse now teaches at Bowling Green State University in Ohio. His father was a minister, so he moved around a lot as a kid, eventually winding up in Oshkosh, Wis., where he graduated from high school. After that came the University of Wisconsin-Madison, but he needed nearly seven years to earn his undergraduate degree because he kept taking time off to make money as a taxi driver. These days, he prefers jogging to driving, and he has qualified to compete in the Boston Marathon this April. He posts running times on his office door. Messer-Kruse is in many ways an ordinary academic liberal. He mentions anti-poverty activist Michael Harrington as an inspiration, calls himself a "social democrat," and says he voted twice for Barack Obama.

In 1986, when he was a senior in Madison, a buddy suggested that they drive down to Chicago for the weekend. "I just wanted to hang out with friends," says Messer-Kruse. His companion also proposed a side trip to a cemetery, where labor activists planned to commemorate the centenary of the Haymarket protest....