But runners across the country can not be stopped by fears of a similar situation. Instead, they dedicate their runs to the 20-year-old student from the University of Iowa and share hopeful news in social media with the #MilesforMollie tag.
"This run is for you, Mollie, we are with you, we will not allow fear to stop us from doing the things we love," wrote Sarah Hannan Bishop of the University of Iowa on Twitter. She put in a picture of her sneakers that have "Miles for Mollie" on the side.
Local running clubs have dedicated their meetings to Tibbetts. Two from Iowa, No Regrets Running and Corridor Running, partner on Sunday to honor Tibbetts.
"Not only did we want to run in their honor, but to show the world that the Running Community is strong and supportive, the group comes from Iowa and Brooklyn, Iowa, is not far from us," No Regret's running member Ara Bungee Ispentchian said.
Several people said it was important to keep motivating young runners in particular.
"Running is rewarding and useful, and I do not want my team, and especially the runners, to lose the joy of this senseless crime." said Alex Oliver, head coach of cross-country teams at Riverside Community School Junior High and High School in Carson, Iowa.
The young runners dedicated their Friday morning workout #MilesforMollie, said Oliver.
Amanda Nethero from Clermont, Florida, said she was part of a local moms RUN This Town group This is about strengthening female runners. "Our motto is that no woman is left behind," she said.
She and her crew ran 10 miles for Tibbetts on Sunday. "No woman should fear for her life every time she straps her shoes," Nethero wrote on Twitter.
"As the mother of a young runner, I've gone miles today reclaiming our roads and paths," said Maria DeAngelo of New York State on Twitter. "I am an athlete and the mother of runners, and I support Miles so Mollie supports women in their right to go to safety wherever and whenever they want."
Men are also part of this effort. Ed Zaniewski from Wellfleet, Mass., Dedicated his Sunday run to Tibbet
"For me, this hashtag is only a small way as a runner, husband and father of a 26 year old daughter to express my support and heartfelt condolences to Mollie's family," said he.
"But for the grace of God I and the thousands of other runners out there are pursuing goals without thinking that they will not make it home."
Others said they ran #MilesforMollie to send supportive messages to their family.
"Mollie Tibbetts just wanted to make a run, because she could not finish, I ran for her yesterday and today," said Tericia Eller of North Carolina on Twitter.
"My motivation to join in is her family, I have daughters, I want them for sure, I want her family to see how much the running community grieves and supports them!"
Wendi Cannon of Tallahassee, Florida, said her #MilesforMollie run was 5.5 miles on Sunday.
"As an avid runner and stroller, my heart was broken when I heard Mollie's story – thoughts and prayers for her family during this difficult time."
Aly Rigg said she was hosting her 11th Half Marathon on Sunday in Reno, Nevada, Tibbetts.
"I've been following their story, and when the news came, it shook the world of running – especially for us runners," she said.
"I ran for them today to show my support to my family, loved ones and all my others. We are a force to be reckoned with."
CNN's Nicole Chavez contributed to this report.