MILWAUKEE — At 18, Julius Burton could not legally buy a gun. So he paid a 21-year-old acquaintance $40 to accompany him to Badger Guns, just outside the city limits here, and be the official buyer of a weapon.

Mr. Burton pointed to a Taurus semiautomatic pistol and said, “That’s the one I want,” according to surveillance video from that day in 2009 and trial testimony. Then he helped his friend, who was struggling to fill out a two-page form. A hovering store clerk helped as well, showing the friend how to correct mistakes and ensure he was listed as the buyer.

A month later, on June 9, 2009, Mr. Burton shot two police officers with the Taurus. One lost an eye and was left with brain damage; the other was seriously wounded in the face.

It was a classic straw purchase, an important way guns enter the underground market, though an unusually well-documented one because of the video and the quick identification of the true buyer.