At first, they thought it was thunder.
Brendan Reilly, president of the Boulder Wave sports agency, was in the dining room of the Fairmont Copley Plaza with many of the elite athletes who had already finished the Boston Marathon, when two bombs exploded near the finish line.
"The first thing everyone did is look up at the sky and think, 'Huh, it doesn't look cloudy,'" he said. "Then we thought maybe it was a cannon for Patriot's Day."
But within a matter of minutes, the scream of sirens made it clear that something was very wrong.
Two bombs near the finish line had exploded, one after the other, killing at least three and injuring at least 144 people, some of them critically.
Reilly wanted to account for his athletes, particularly Nuta Olaru, a Romanian-born runner who now lives in Longmont. She had left the hotel to watch more of the race and do some shopping. She was roughly 100 meters from one of the explosions but returned safely to the hotel, which houses the invited athletes and serves as race headquarters.
The hotel was put on lockdown, and most of the athletes spent the day answering text messages and voicemails from worried friends and relatives around the world.
Reilly, who lived in Boston for 10 years before he moved to Colorado, said it was jarring to see twisted debris and emergency responders on Boylston Street, which normally presents a scene of celebration after the marathon.
"Who in the world would attack an event that is about nothing but good will and international friendship?" he said. "Nothing but good feelings come out of the Boston Marathon. What were they were trying to get our attention about? ... It's very, very sad."
Dozens of Boulder runners and their friends and family members were in Boston for the marathon. The marathon's athlete tracker shows 61 participants registered from Boulder and at least another 41 from other Boulder County cities and towns. There were 512 runners from Colorado registered in the race.
Boulder-based members of the running media also were at the race.
Brian Metzler, a Boulder running journalist and editor in chief of Competitor Magazine, had just left the media center and was looking for finished runners when he heard the blast. It sounded like muffled thunder because there were several buildings between him and the explosion, he said in an email.
"Then I heard a second one and saw people running in all directions," Metzler wrote. "Several of my colleagues were out there, so my immediate thought was where they were and if they were OK. The scenes I saw were horrific. People were bloody and crying and in complete shock."
Lorraine Moller, a Boulder-based distance runner and previous winner of the Boston Marathon, described the bombing as "sacrilegious."
Moller had planned to go to Boston this year but changed her mind at the last minute. She had secured finish-line passes for some friends, and her first thoughts turned to them.
Her friends are safe, but she is thinking of those who were not, Moller said.
"It seems that innocence itself, what we runners signify, the purity of movement and the feeling of your own power and being able to achieve something, has been tampered with, and that is sacrilegious to me," she said. "I feel very sad."
Boulder-based distance runner Jason Hartmann had finished fourth in the race nearly two hours before the explosion. He was getting drug-tested at the time of the blast and didn't hear it. His first indicator that something was wrong was that he couldn't reply to texts coming in to congratulate him. He turned on the TV and saw the coverage of the explosion and its aftermath.
"If you're a runner, it's just one of the pinnacle races that you can do and everyone wants to do," he said. "Whether you're spectating or running, the crowds are wonderful. The whole experience here is wonderful. To have a tragedy like this happen is just so sad."
Mark Bockmann, of Boulder, learned about the bombings on Facebook later in the afternoon after he had left the race.
He said he had posted on his Facebook page to tell his friends that he'd had a poor race because he walked a portion around mile 17. Then one of his friends replied that he was glad Bockmann was OK, having heard about the bombings. At first, he said, he ignored it.
"It was just kind of a shock," he said.
After a while, Bockmann said he crunched some numbers and said if he had continued to walk the race, as he had wished, he would have put himself at the finish line around the time of the explosions. He is grateful for his second wind.
Jessica Bianco, a Boulder IT nurse, was running in her fourth Boston Marathon and was about three-quarters of a mile from the finish when the explosion occurred. She didn't hear it because she was in an underpass. As she emerged with her friend and her fiance, who had been running with her from the 23-mile marker, a police officer stopped the race. At first, no one knew what was going on, but it was so unusual that they knew it was bad. Soon, word spread of the explosion, but it was some time before instructions came for how to leave the race route.
Bianco said she was impressed with all the people who came out with food, plastic bags to use for warmth and offers of cell phones.
"I kept thinking I was so grateful I was with my friend and she wasn't alone and (my fiance) was with me," she said.
Bianco runs on behalf of the Dana-Farber Cancer Institute, where her father was treated before he died, and the Boston Marathon holds special meaning for her.
"It's so sad that there are people out in the world who do this," she said.
With cell phones down for much of the afternoon, people used social media and text messages to let loved ones know they were OK.
The University of Colorado's athletic department said in a tweet that former Buff runner Kara Goucher, who finished sixth, was safe.
"My heart is broken, but my family and I are fine," Goucher, who now lives in Duluth, Minn., tweeted later in the day. "Please pray for those who aren't."
Bill Sullivan, the father of Sara Sullivan, a 2006 Longmont High School graduate who completed the marathon, said his daughter was safe but was close enough at the time of the explosion that she had to take cover with her boyfriend.
"She's fine, but there are a lot of people out there hurting right now," Bill Sullivan said.
Steven and Susan Kohuth, of Superior, were two blocks away in the family waiting area with their daughters, 9 and 14, when the bombs exploded. Steven Kohuth, a counselor at Denver East High School, had just finished his third marathon and his first in Boston.
He has been running marathons for six years, Susan Kohuth said in a text message interview, and he was proud to qualify for Boston.
"It's every distance runner's dream," Susan Kohuth said of the Boston Marathon. "Up until 3 p.m. today, anyway."
Geralyn Eastman, of Superior, said the first bomb went off 12 seconds after she crossed the finish line.
She wasn't injured, but her spectator husband, Grady, was close to where one of the bombs exploded and has an abrasion on his leg. His ears were still ringing Monday night. A person next to him lost a foot, she said.
"Angels were with us," she said.
Longmont Times-Call reporter P.J. Shields and Camera Staff Writer Amy Bounds contributed to this report.
Contact Camera Staff Writer Erica Meltzer at 303-473-1355 or email@example.com.