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Some 30,000 runners are spending Sunday making their final preparations for Monday's Boston Marathon.
While many of the runners will be racing in memory of someone, for one group, that loss is especially raw: They are lacing up for Dr. Michael Davidson, a surgeon at Boston's Bringham and Woman's Hospital, who was tragically killed four months ago.
"He truly, truly cared, and was intensely invested in his patients," Dr. Charles Nyman told TODAY's Erica Hill about his friend and colleague.
On Jan. 20, Davidson — who colleagues say was known for spending extra time with his patients — had spent nearly 40 minutes with the son of a former patient, who was distraught over his mother's death.
But Stephen Pasceri had come to Davidson's hospital with a plan -- and a gun.
Pasceri shot Davidson twice and then took his own life. The hospital's staff, including Nyman, tried to save Davidson's life.
"It was probably the hardest day of my professional career," Nyman said. "I think about it more than I'd want... I hope, you know, with the running, that I'll think about it less."
Nyman is one of 10 colleagues running the Boston Marathon in memory of Davidson. The group said they came together quickly, with barely enough time to train, but were anxious to find some way to honor their friend and supper his family.
Davidson's wife, Terri, was pregnant with the couple's fourth child when he was killed. The baby girl, named Mikaela in honor of her father, was born two weeks ago.
"You would like to think that if something like this happened to you, that a community would come together and help take care of your family," Dr. Pinak Shah, who used to work with Davidson at Bringham and Woman's Hospital, said. "That's a huge motivation for us to do this."
Davidson's father will be in Boston, watching along the route on marathon Monday, just as he was five years ago when his son ran the race to celebrate his 40th birthday.
"I'd like to think that Michael is watching and is very grateful for what they're doing in his memory," Robert Davidson said.
The team has raised more than $70,000 for Davidson's family, with donations coming in from around the world.
"Here's a guy who's trying to do good...who's doing what he loved to do. And for his life to be taken in this manner is just, it's unfathomable," Shah said. "I think it's easy for people to want to try to do something to help."
At the hospital, there are daily reminders of how much Davidson touched the community — people wearing hearts with Davidson's initials in the middle.
"I think it's a testament to how much this tragedy has touched the entire institution," Shah said. "It makes me feel good that people are constantly thinking about him."