The weekend before he was murdered, UNC dental student Deah Barakat placed a surgical mask over his face, hoping it wouldn’t frighten the 4-year-old boy in front of him.

The boy, wearing a red Spider-Man T-shirt, gazed curiously at Deah, who smiled behind the mask.

Deah, clad in blue scrubs and a friendship bracelet, laid out four Sesame Street stickers, allowing the boy to pick his favorite. Then the 23-year-old Deah displayed two toothpick-sized rods, each with an oval sponge at one end.

“These are lollipopsone for me and one for you,” Deah pretended. “Now, I’m going to count your teeth with my lollipop, but I need you to hold onto yours as a backup.”

The boy was most likely a family member or acquaintance. Deah was using him as a practice patient, in preparation for field work in March. The work involved collecting saliva samples from young children to determine if they had a genetic disposition to cavities. The exchange with the boy was captured in one of the last photographs taken of Deah before a gunman executed him Feb. 10 alongside Deah’s newlywed wife, 21-year-old Yusor Mohammad Abu-Salha, and her sister, 19-year-old Razan.

There are lingering questions about the tragedy. Questions involving parking and vigilantism. Mental illness and gun ownership. Religion and hate. But in reflecting on the triple-murder, some have preferred to focus on the things that are certain. Things like kindness, hard work, and the power of a smile.

Deah had enrolled in UNC to help poor children without dental care. For the past year he had provided free screenings, supplies and food for homeless people. His plan was to open a practice with Yusor. They were young and in love.

The boy in the Spider-Man T-shirt opened his mouth. Carefully and tenderly, Deah swabbed the boy’s gums and tongue, pretending he was counting his teeth. “One … two… three,” Deah said, as he prodded the boy’s mouth for saliva pools, periodically squeezing the droplets into a receptacle for DNA extraction.

The project was part of a larger one run by Deah’s UNC adviser, Kimon Divaris, who had already collected hundreds of samples from North Carolina children. In a first-of-its-kind study, Divaris’ research group is identifying the biological causes of oral decay in preschool-aged children.

Deah had asked Divaris to assist. The two men quickly realized they were kindred spirits. They shared the same interest in community service, public health, and pediatric dentistry. They both loved basketball. After Deah joined the research team, he often stopped by Divaris’ office to discuss the Milwaukee Bucks.

Both men also had back problems. When Divaris complained about his, Deah recommended his chiropractor. One afternoon, when Divaris walked into his office, he discovered a gift from Deah: a plastic back scratcher. It came with a note: “To the best mentor.”

Deah recently had shifted his attention to global health. He had already served on a dental aid mission in Palestine, and he was raising money for a trip to Turkey, to provide services and supplies to child refugees from war-torn Syria. In a YouTube fundraising clip, Deah appears in front of the camera, explaining that with proper care, the Syrian children’s lives can improve. “Let’s relieve their pain,” he says.

Last year, Deah completed his first journal-qualified paper, in which he analyzed the data from Divaris’ saliva samples. Deah studied more than 200, determining which ones generated accurate DNA information. He also assessed the feasibility of taking saliva samples from children, who are often too anxious or preoccupied to handle a “lollipop” swishing inside their mouths.

Earlier this month, Deah visited Divaris. The data analysis was going along fine, said Deah, but he wanted to join Divaris in the field, collecting kids’ saliva samples at schools, churches and Head Start programs. Divaris, who had already welcomed the idea, handed Deah a saliva-extraction kit. “See if you can find a friend or family member to practice on,” he said. Deah left the office thrilled.

During the next few days, Deah and Divaris exchanged a series of emails. “Let’s chat more,” Divaris wrote in an email on Tuesday. “Gotcha!” replied Deah at 2:20 p.m. His signature included a photo in which he flashes his contagious smile.

Three hours later he was dead.

More than 5,000 mourners traveled to a N.C. State soccer field to view the caskets of Deah, Yusor and Razan. They were buried in an Islamic cemetery in Wendell.

Since their deaths, hundreds of thousands of dollars have flooded a fund to build new dental clinics in Turkey, Jordan and Raleigh in their honor. A clinic in Reykxanly, Turkey, built for Syrian refugee children in Deah’s name, opened last Thursday.

This week in his office, Divaris memorialized his late student. “We’re already trying to think what he would like us to do: engage in the community, make a positive difference, and do it quickly,” the professor said. Deah’s legacy will inspire students for years to come, he added, pausing once to choke back tears. Nearby, the back-massager Deah gave him hung on a shelf.

Deah will be remembered for many things: His hugs. His enthusiasm. His diligence. His maturity. His ability to identify a need and find a practical solution.

But one thing seems to stand out. “Deah’s smile is what most people remember,” said Divaris. And though it was short-lived, it’s fitting that he entered a career dedicated to brightening those of others.

This article appeared in print with the headline “A million-watt smile.”