CHICAGO  — Far-flung family members, co-workers and friends frantically used social media, cellphones and even a “people finder” website Monday to try to learn the fate of participants and spectators at the Boston Marathon, where three people were killed and dozens injured after a pair of bombs exploded near the finish line of one of the world’s great races.

The search was made more difficult because heavy cellphone use caused slow and delayed service. In an age connected by everything digital, the hours after the blasts produced a tense silence.

At the race, 51-year-old Julie Jeske, of Bismarck, N.D., had finished about 15 minutes before the explosions and was getting food about two blocks away when she heard two loud booms. She immediately tried to call her parents but could not place the call. A friend was able to post on Facebook that they were OK, but reaching her parents was another worry.

“I wasn’t able to call and I felt so bad,” Jeske said. “When I was finally able to reach them, my mom said she was just absolutely beside herself with fear.”

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 safe 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. A few hours after the explosion, the site indicated it was tracking 3,600 records.

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 Co. 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.”

Kim Hauser, a substitute teacher in the Chicago area, did not know about the explosions until her students went home and she got a chance to look at her phone. There were messages from acquaintances asking, “‘Is your brother OK?” She searched the news and it dawned on her why they were asking.

“I tried to call him immediately, but there was no cell service,” the Frankfort, Ill., woman said. “I waited anxiously by the phone. I just felt horrible. I had a hard time holding myself together.”

Five minutes became 10, then 20 — finally, 45 minutes later she looked down to see a text from her brother, Thomas Wiora. He had crossed the finish line shortly before the explosion and was 120 yards away when it went off. But he was fine.

“I was relieved,” she said. “But the whole thing was so heartbreaking.”

Mary Butler, of Oklahoma City, hadn’t been able to reach her husband, Jason, 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.