The search was made more difficult because heavy cellphone use caused slow and delayed service.
Jan Seeley, director of the Illinois Marathon in Champaign, Ill., said she spent much of Monday afternoon trying to reach the runners from her area who she knew were at the race. She reached most of them, but still was waiting to hear from a handful of others.
"I've left messages for everyone I know," she said. One woman she knew crossed the finish line just a minute or so ahead of the explosions.
Tim Apuzzo of Seattle said he spent an agonizing 10 minutes frantically trying to call his girlfriend, Quinn Schweizer, who was watching the marathon with her friends at the finish line. But when he kept getting a recording saying there was no service, he started to worry "because you know you have a group of people in this generation all wired in ... and quick to respond."
Finally, she was able to call him to say she was fine and that her group had left the finish line just minutes before the blast to walk to a cafe for lunch.
Google stepped in to help family and friends of runners find their loved ones, setting up a site called Google Person Finder that allows users to enter the name of a person they're looking for or enter information about someone who was there.
Mary Beth Aasen of Shorewood, Wis., and her husband were using an app to track their daughter Maggie's progress along the marathon route. They didn't realize anything was wrong until a worried friend texted Aasen and asked if Maggie was OK.
The app indicated that Maggie was still moving, a relief for her parents. Mary Beth Aasen tried in vain to call her daughter for about 30 minutes before Maggie called her.
"When I talked to her she was pretty upset," Aasen said. "Physically she said she felt great but she was upset because she hadn't been in contact with her friends."
Aasen said she was waiting for Maggie to call her back with an update, but knew cellphone service was slow in the area.
"I just feel terrible for the people who haven't been in contact with their family and friends who are there," she said. "I'm praying for everyone who hasn't heard yet."
David Meixelsperger, who owns the Berkeley Running Company in Madison, Wis., finished the race about 90 minutes before the explosion. He sent an email to customers of his store and friends in the running community letting everyone know he was safe, but that he couldn't send or receive calls on his cellphone.
"At this time, all Berkeley Runners and Customers are safe," he said in the email. "We have been texting each other to seek out their whereabouts."
Mary Butler of Oklahoma City hadn't been able to reach her husband, Jason Butler, who was running with his son, brother and other family members. But she said he'd posted on Facebook that he and the others were OK.
"That's all I know about it," Mary Butler said, adding she'd been trying to call since she'd first heard of the explosions. "I'm just waiting -- keep trying to call."
She declined to talk further so that she could keep her phone line open.
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