Last year, the cross-country runners were volunteers at the starting line.
"We were at the starting line helping all the runners, putting their bags on the busses," Karstunen said. "Then we headed back to school for practice. Once we got back, that's when we heard the news."
Two homemade bombs exploded near the finish line, killing three people and injuring dozens.
One of the victims was 29-year-old Krystle Campbell, a former UMass-Boston student. The two students will run in her name.
"It's hard. It's hard whenever someone has to lose their life," Laurencelle said.
The two are also trying to raise $2,500 to give to the Krystle Campbell Scholarship Fund. They're selling $10 T-shirts to meet the goal.
Neither of them knew Krystle, but they'd rather honor her than give any thought to the UMass-Dartmouth freshman tied to the bombings.
Days after the marathon, the entire campus was shut down as authorities learned Dzhokhar Tsarnaev, one of the suspects, was a student.
"It actually kind of hit closer than I thought it would," Karstunen said. "I actually lived in the same dorm as him."
It's why students and staff gathered Wednesday around the university's section of the America 4 Boston Prayer Canvas.
"I wrote my name and UMass-Dartmouth Strong," said Chancellor Divina Grossman. "That's how I feel about how the campus responded to the aftermath. We were strong because we were united as a community."
On April 21, Karstunen and Laurencelle will represent that community.