He was talking with the people waiting in line Saturday to see U.S. Rep. Gabrielle Giffords when a gunman opened fire.
"He was asking them what they needed," Mark Kimble, a speechwriter standing nearby, told The Arizona Republic.
Zimmerman was one of six people killed in the rampage, along with U.S. District Judge John Roll, 9-year-old Christina-Taylor Green and three Arizona retirees, Dorwan Stoddard, Dorothy Morris and Phyllis Scheck, who were outside the supermarket where the event was taking place.
Zimmerman, 30, was Giffords' community outreach director. Engaged to be married, with a master's degree in social work, he had worked for Giffords since her first congressional campaign in 2006, starting as a field organizer, The Arizona Daily Star reported.
"He also organized town halls. He was an amazing person. His goal in life was to do nothing but help people. There's not enough words to describe him," Giffords' press secretary, C.J. Karamargin told the Sierra Vista Herald/Review.
Zimmerman helped Giffords' constituents with their daily troubles, assisting those who, for instance, needed help with their Social Security checks. He also was in charge of logistics for her public events, such as Saturday's.
"He would go out of his way to help people in trouble," Daniel Graver, who worked for Giffords as a legislative assistant, told the Daily Star. "People would come into the congressional office, he would listen to them and give them money for a cab home."
Zimmerman's father, Ross, told the newspaper his son had a gift for working with people since childhood. A runner who had hiked the Grand Canyon from the North to South Rims twice, he was engaged to marry a Tucson Medical Center nurse, Ross Zimmerman told the newspaper.
Among the others gathered at that corner outside a supermarket were Dorwan Stoddard, 76, and his wife, Mavanell, who lived just a mile from the grocery store. Both were shot – he was fatally shot in the head and she was hit in the legs and is expected to recover.
Dorwan Stoddard was a former construction worker and the man his church turned to when something needed fixing, Mike Nowak, their minister at Mountain Avenue Church of Christ, told the Daily Star. He also organized the benevolence committee, the group that reached out to the poor.
"He always gave of himself and never asked for anything in return," Nowak told the newspaper.
Friends said the couple had been high school sweethearts who fell in love again after their spouses died.
"He got on top of her and tried to shield her," Jessica Knapp, who works with the youth group at the Mountain Avenue church, told the Chronicle.
She described the Stoddards as "the lifeblood of our church."
"If the baptistery was leaking, we called Dorwan," she said. On the benevolence committee, she said: "Instead of just writing checks, they would go meet with people and find out what was going on in their lives and what their needs were. ...
"This is going to be a huge hole in our congregation," Knapp told the Chronicle.
Avid travelers, the Stoddards left Arizona every April as the weather got hot, returning every October, friends told the Daily Star. They had traveled to all 50 states, and visited 28 countries, friends said.
The tragedy's youngest victim, 9-year-old aspiring politician Christina Taylor Green, was born on Sept. 11, 2001. She had gone to the Safeway supermarket to watch Giffords in action because the girl had just been elected to her school's student council. "She came in on a tragedy and now she's gone out on a tragedy, but the nine years in between were very special," her father, John Green, told CNN.
Christina was the granddaughter of Dallas Green, former manager of the Phillies, Yankees and Mets. Her father is a scout for the Los Angeles Dodgers organization.
Victim Judge John Roll was appointed to the federal bench in 1991 by President George HW Bush after serving as an Arizona state attorney and a state court judge.
"He was a life-long Republican and I don't think he would have objected to hearing anyone call him conservative," Michael Daly Hawkins, a judge on the federal Ninth Circuit Court of Appeals in Phoenix, told The Wall Street Journal. "But he was always fair-minded and absolutely attentive to everyone who appeared in his courtroom." Roll is survived by his wife, Maureen, three sons and five grandchildren.
Details were beginning to emerge Sunday about the others killed in the tragedy. Phyllis Schneck, 79, was a New Jersey grandmother who retired to Arizona, according to The Trentonian. Though Schneck was a lifelong conservative, she voted for Giffords, a Democrat, and was attending Saturday's event to meet the congresswoman.
Dorothy Morris, 76, was attending the event with her husband, George, who remains in critical condition, according to the Reno Gazette-Journal. Like the Stoddards, the couple were high school sweethearts. They have two daughters.
President Barack Obama has ordered a moment of silence at 11 a.m. EST on Monday for the victims of the shootings.
"I call on Americans to observe a moment of silence to honor the innocent victims of the senseless tragedy in Tucson, Ariz., including those still fighting for their lives," Obama said in a prepared statement. "It will be a time for us to come together as a nation in prayer or reflection, keeping the victims and their families closely at heart."