Just around the corner from the Boston Marathon finish line, Erick M. Juarez ’15, one of more than 90 Harvard affiliates who attended the Marathon, was calmly walking back to Harvard’s campus at 2:50 p.m. on Monday after watching the race.

But then the ground shook.

At first, he thought the construction crew working on a building down the street had dropped a heavy object. As smoke billowed overhead and shouting runners and spectators sprinted past him, Juarez realized that the truth was far more sinister. Turning around, Juarez made his way towards the source of the chaos, fueled by both an “innate curiosity” and a desire to help, he said.

Arriving at the intersection of Boylston and Exeter Streets, Juarez witnessed paramedics carrying stretchers with bloodied runners and spectators—a grim scene caused by two explosions moments earlier. Police officers shouted over the wails of crying children. Ambulance sirens soon blared overhead, adding to the hysteria as more people fled the site of the bombings.

“I just stayed there for a little while watching, trying to absorb, thinking that it could have been me,” he said.

Three people died and more than 130 were injured, though as of press time there were no confirmed reports of Harvard faculty, students, or staff suffering injuries due to the blast.

The disaster was an unexpected twist to a historic day that has traditionally been an opportunity to celebrate.

Dennis J. Purcell, Jr. ’13 was excited for the 117th Boston Marathon to be his first.

“Everyone’s spirits were up,” he said. “It was just a fun day.”

Purcell had nearly reached the 25th mile marker when he heard mention of an explosion from runners and spectators. Thinking nothing of it, he continued his race. But with only one mile left, his mother and sister emerged from the crowd and frantically guided him off the course.

Like Purcell, many other Harvard affiliates felt disoriented by the thundering blasts.

Hearing the sound of explosions, Kendall P. Crawford ’16 thought celebrations of the race’s finish might include the firing of a cannon.

Crawford, who waited near the finish line to support a fellow Harvard student, did not realize anything unusual had occurred until a second explosion erupted behind her.

“After the second one I think [it] really hit me that ‘oh my god I could die,’” Crawford said. “‘This is big.’”

Though he had finished the race when the bombs exploded, competitor Beau M. Feeny ’13 said he heard the blast from a friend’s apartment on Dartmouth Street.

Feeny said he immediately thought of the friends who had come to watch him in the race.

“It was terrifying,” he said. “I knew that I had asked a bunch of friends to come and watch me and they weren’t accounted for.”

Spectators at the finish line reported screaming children, blaring ambulances, and mass hysteria as people fled the scene.

Crawford said she saw other spectators tearing down the barricades separating the bystanders from the runners as they attempted to flee the finish line area.

“My friend and I just grabbed each other and stood there in absolute fear,” she said.

As Juarez returned to the scene of the explosion, he witnessed a chaotic mass of people, some frozen in confusion, others fleeing the streets.

“There were a lot of children crying, which really got to me,” he said. “It was very shocking, very out of place. It made me wonder why stuff like this happened.”

Harleen K. Gambhir ’14, who attended the race as a spectator, approached the finish line as the bombs exploded.

“I had a moment when I thought that everything might blow and there would be nowhere to run to,” she said. “A woman near me fell immediately and started screaming and crying.”

Many terrified Harvard runners and spectators expressed gratitude that they remained unscathed.

“I’m really thankful that I hadn’t run it a little faster,” Purcell said. “Unfortunately I couldn’t finish, but it could have been a lot worse.”

Juarez said he had considered going to the marathon’s finish line to take a photo, but ultimately he had decided against it.

“Had I made my way there a few minutes earlier I might have been a victim,” he said. “I thank God that that didn’t happen.”

With streets and nearby Massachusetts Bay Transportation Authority stations closed and taxis at a premium, Harvard affiliates encountered difficulty as they tried to make their way back to campus.

Feeny was among many who said they walked most or all of the way from Back Bay to Cambridge.

Harvard University Police Department officers and Gene A. Corbin, assistant dean of student life for public service, offered rides to students stranded in downtown Boston.

Many Boston-based alumni of the Harvard College Marathon Challenge, a program that allows helps students secure a spot in the race, offered their support to Crimson runners, according to Craig F. Rodgers, a counselor at the Bureau of Study Counsel who also coordinates the Challenge.

“We are blessed to be part of such a wonderful, close community here at Harvard,” he said.

—Staff writer Laya Anasu can be reached at layaanasu@college.harvard.edu. Follow her on Twitter @layaanasu.

—Staff writer Elizabeth S. Auritt can be reached at eauritt@college.harvard.edu. Follow her on Twitter @eauritt.

—Staff writer Dev A. Patel can be reached at devpatel@college.harvard.edu. Follow him on Twitter @dev_a_patel.