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.