TUCSON, Ariz.—Arizona federal Judge John Roll appears to have died while saving the life of another man during the shooting rampage here on Jan. 8, according to an investigator who has viewed surveillance video from the crime scene.

Video from the Safeway supermarket near Tucson shows the accused gunman, Jared Loughner, walking past a concrete pillar, raising his gun and firing at close range directly at U.S. Congresswoman Gabrielle Giffords, who is standing off-camera, said Chris Nanos, a captain with the Pima County Sheriff's Department who has viewed the video.

Only a wisp of what appears to be Ms. Giffords' hair is seen on the video, Mr. Nanos said. The gunman fires first at Ms. Giffords "without hesitation" then "fires indiscriminately at the crowd," he said. The group had assembled to hear Ms. Giffords at a planned constituency event that day.

"He wheels this direction and that direction firing," Mr. Nanos said. The shooting killed six people and wounded 13 people, including Ms. Giffords, who is in serious condition at a Tucson hospital after a bullet passed through her brain.

As the shooting starts, the video shows Judge Roll pushing another man, Rob Barber, onto the ground, Mr. Nanos said. "It looks to us as though he is pushing against Ron Barber to move him out of the way." Both men fall to the ground; both are shot. The judge was shot in the back and died.

"It's pretty evident to me that Judge Roll was a hero … if Judge Roll had not pushed Mr. Barber his wounds might have been fatal," Mr. Nanos said. "Judge Roll's actions are of a man trying to save another man's life."

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