TUCSON, Ariz. » What if he had not had that second cup of coffee? What if he had not asked the cashier about the two-for-one special on cigarettes? Maybe he would have been there a minute, or just 30 seconds, earlier. Maybe that would have been enough.

Joseph Zamudio was like any of the other witnesses to the Jan. 8 shooting rampage here — terrified, bewildered, furious. But he had a gun with him that day, when a young man opened fire, killing six people and wounding 13, including Rep. Gabrielle Giffords.

So now Zamudio wakes up at night breathless, unable to fall back to sleep, torturing himself about whether he might have done more that morning to stop the gunman.

As victims and witnesses in the shootings replay the day in their minds, some — like Zamudio, who was there to buy cigarettes and ended up helping restrain the gunman — ask themselves what they could have, maybe should have, done differently.

What if Giffords’ aides had requested security? What if the bystanders had been quicker to tackle Jared L. Loughner, the 22-year-old accused in the shootings? What if they had just gone somewhere else that day? Why did they live when others, standing just inches away, had died?

Psychologists have a term for it: "survivor’s guilt."

"They have to call it something," said Zamudio, who has been seeing a counselor regularly since the shooting. He finds that talking helps.

So does Suzi Hileman, who days after the shooting awoke in her hospital bed shouting: "Christina! Christina!"

That Saturday morning, Hileman picked up Christina-Taylor Green, her 9-year-old neighbor, and promised the girl’s mother that they would return in three or four hours.

Hileman, 59, had simply wanted to take Christina to meet their congresswoman. They would make a day of it — going for lunch and a manicure after the "Congress on Your Corner" event outside a local Safeway. Instead, a gunman opened fire, killing Christina and wounding Hileman.

"I never got to bring Christina home," Hileman said. By now, her voice is almost matter-of-fact. But her sadness is betrayed by the long pauses she takes, the way she buries her face in a throw pillow when the tears start to fall.

The guilt comes in waves. It was there in the hospital. It still lurks, threatening to return at any moment. When someone asks about it, she calls her husband over to hold her hand as she answers.

For Hileman, the rawness of the guilt has worn off, along with her pain medication. As she sat on her couch last week, the evening after returning home from the hospital, she raked her hands through her cropped salt-and-pepper hair and repeated the same thing she had said to herself for days and days.

It almost sounds like an affirmation: "I am a woman who took a little girl to the market," Hileman said.

It is the ordinariness that confounds her. "I don’t feel guilty about that," she continued. "I can feel bad about what happened, but I can’t feel bad about being there. What happened had nothing to do with Christina and me and why we were there."

Survivor’s guilt is intensely complicated and personal. Randy Gardner, who was shot in the foot that morning, wonders if he could have done more, somehow, rather than running to protect himself. Ronald Barber, who leads Giffords’ district office, has asked himself countless times why he survived while two of his friends, standing on either side of him, did not.

Dr. Jeffrey Lieberman, a professor and the chairman of psychiatry at Columbia University Medical Center, said that this sort of traumatic experience inevitably changes people, creating "soul-searching moments."

"None of the people that were there will ever be the same," Lieberman said. "The question is how they will handle this. Will they grow and use this as a positive psychological adaptation? Or will it gnaw at them and be a memory that gives them emotional distress?"

Perhaps nobody asks himself more questions than Zamudio, 24, who arrived at the Safeway parking lot just as the shooting stopped, his gun tucked inside his jacket.

He did not use it, did not even pull it from his pocket. He had been in Walgreens, buying Camel cigarettes and asking about the two-for-one special. By the time he ran to the Safeway, the shooting was over. People were on the ground, already dead, or close to it. Now, the weight of those 30 seconds can be crushing.

"Maybe he could have only got through half his clip if I had gotten there in time," Zamudio said, his voice flat and eyes distant. "I shouldn’t have been there buying cigarettes. I should have been there to shoot him."

Hileman, a former social worker, says asking "What if?" is a waste of energy. But she knows that the question could come racing into her mind at any second. She has not spent much time alone since the shooting; she has been constantly surrounded by her closest friends, her daughter and her husband.

So Hileman is dealing with her feelings the best way she knows — she is talking to anyone who will listen. ("I’ll be talking in my grave," she told a visitor.) For years, she worked in hospitals and rehabilitation centers, helping others cope with tragedy.

When Hileman was still in the hospital, after being shot three times and shattering her hip when the bullets knocked her down, she said the biggest challenge to her recovery would be getting past the guilt.

But in many ways, she is among the most sympathetic characters in the aftermath of the shooting. She is the woman any mother can identify with. She said she was "overwhelmed by the outpouring of love" from friends, neighbors, even strangers.

With her adult children hundreds of miles away and no grandchildren, Hileman has taken several youngsters in the neighborhood under her wing. With Christina, she bonded over games of pickup sticks. And she saw herself in the child.

"She was a little girl who had a little curl, right in the middle of her forehead," Hileman said. "We were definitely a couple of partners in crime."

Now, Hileman said, "I’m just really, really sad. I lost a friend. My girlfriend died 10 days ago. How would you feel?"