As prosecutors build their case against the man accused in the mass shooting that critically wounded an Arizona congresswoman this month, a key element will be the story of Ron Barber.

The congressional staffer was standing at Rep. Gabrielle Giffords's side in a Tucson parking lot Jan. 8 when a gunman opened fire, shooting Giffords in the head and then firing bullets into the crowd. One struck Barber's left cheek, exiting from the back of his neck. Another ripped through his left thigh.

On Wednesday, 22-year-old Jared Lee Loughner was charged with attempting to kill Giffords, Barber and another staffer, Pam Simon. It is a first wave of charges expected in connection with the incident that wounded 13 and killed six, including a third staffer, community outreach director Gabe Zimmerman.

Barber, 65, a state government retiree who had found a second career working for Giffords as her district director, said in a telephone interview Thursday that he remembers the morning of the shooting crisply. He had first stopped by Giffords's office to grab some last-minute materials for "Congress on Your Corner," a regular event Giffords held to hear from her constituents.

By the time he arrived in the Safeway parking lot close to 10 a.m., everything had already been set up. The tables, along with some old velvet rope posts that Zimmerman had picked up at a store that was going out of business, were arranged in a sort of corridor to allow constituents to walk one by one to meet the congresswoman.

The first person to come up, he said, was an Army reservist who had wanted to show Giffords his commendation from Iraq. Next came a couple. He recalled chatting with U.S. District Judge John M. Roll, who told him he had come to thank the congresswoman for her help on a judicial matter. Roll's presence on the scene for official business will be critical for the federal case against Loughner.

It wasn't long before a young man Barber hadn't noticed in the crowd strode up to the end of the line, raised a gun, and shot Giffords in the head.

"It's a bit of a blur, but I saw a man with a gun come in to our little exit," Barber said. "He came in that area, between the two tables. His gun was drawn, and I saw him shoot the congresswoman. Almost right away, he shot me. I went down."

Authorities say they have watched this narrative unfold in security tapes recovered from the scene. Though the tapes have not been released publicly, an official with the Pima County Sheriff's Office has said the footage shows Roll covering Barber with his body to shield him from the onslaught. Roll was killed.

Barber said he does not recall Roll's gesture but would not be surprised, "knowing John, what kind of man he was, how selfless he always was in our dealings with him."

He does, however, remember Safeway shopper Anna Ballis putting pressure on the leg wound that was gushing blood.

Barber said he has wracked his brain to think if he remembers Loughner from a 2007 "Congress on Your Corner" event at which Loughner was apparently dissatisfied with an answer Giffords gave to a question he asked - an incident that friends say marked the start of his focus on the congresswoman. But Barber can't remember the encounter.

He said the past two weeks have brought a complex mix of emotions. Barber has been struck by the outpouring of support from the community, but troubled by the senseless loss of life. He said his feelings hollow out when he considers the shooter himself: He feels nothing toward him.

The wound on his face is now covered by a small bandage, and his foot is numb. Doctors say the feeling will likely come back.

"I'll be back at the office as soon as this foot gets better," he said. "I would do it right away, but I don't think anyone's going to let me."

And then there is this: "The other thing that troubles me, and I know I'll have to deal with this as time goes on, is that I survived, and on either side of me the outcomes are so different. How do I deal with that?"