By Zack Sampson and Emma McGrath, News Staff

Jennifer Li was steps away from the first bomb.

There was a fleeting moment of stunned silence. Smoke rose in the air and people fell to the pavement, bleeding.

A sophomore chemistry major, it was Li’s first visit to the Boston Marathon, a hallmark of the city’s Patriots’ Day celebration each year. On Monday, two explosions rocked the finish line on Boylston Street, killing three people and injuring more than 170 others.

In a letter to the Northeastern community, President Joseph E. Aoun said three Northeastern students were injured in the two explosions, which came 12 seconds apart. None of the students suffered life-threatening injuries.

Li was standing on Boylston Street at 2:50 p.m., when the first bomb – a pressure cooker filled with nails and BBs – sent shrapnel flying into spectators there to cheer on the runners.

“We thought it was a celebration kind of thing because that’s what you would expect at a finish line,” Li said of the initial blast.

Then came another explosion further down the road. Screams became the soundtrack to chaos. Li said that the woman standing directly next to her was bleeding from shards of glass that had hit her leg. Others had their limbs completely torn off in the blast.

“People were just bleeding all over the place,” Li said.

She managed to flee uninjured to a nearby restaurant.

The bombings, deemed an act of terrorism, have resonated across the world. The FBI is leading the investigation into the explosions, and no suspects have been arrested yet, according to authorities. In Boston, the attack has unified the city and strengthened the resolve of its people.

For many Northeastern students – those who ran, watched from the sidewalks and volunteered at the finish line – the event has left an indelible mark.

Brittany Young, a senior human services major, had just crossed the finish line on Boylston Street little more than a minute before the first bomb went off. Within seconds, the joy of finishing her second Boston Marathon running for the Boston Bruins Foundation turned to shock.

“Boom within a minute and thirty seconds, to be like, ‘I’m lucky to be alive,’ it’s just really hard to stomach because like, why am I alive, why do I have my limbs?” she said. “With so many people injured, it’s just like, why am I lucky?”

Young said the explosion shook her, and she turned to see a plume smoke. Fellow runners stood bewildered, wondering what had just happened. She said she tried to tell people to calm down, breathe and grab their medals as marathon volunteers directed them away from the area.

“One of the runners next to me said, ‘I would, but I can’t, my mom’s right there,’” Young recalled.

She stopped. Her mom, too, had been across the street from the first bomb.

“It was impossible to hold back the tears, they just started streaming down my face,” Young said.

Her mom had begun to walk toward the finish after Young completed the race, so she was safe, but Young said the what-ifs of the marathon still bother her. Nevertheless, she said the city should take pride in its fortitude after the attack and she plans to run the marathon again next year.

“The air and the energy, I think we need to clear the clouds out and just realize that through so much darkness we can find light, and not to be corny, but it is true,” Young said.

Among the volunteers who helped Young and her fellow runners at the finish line were nursing students Mary Solomon and Allison Gould. Solomon worked in Medical Tent A, which served as a triage site after the blasts. Gould was stationed at nearby Medical Tent B.

Solomon said she was also at Medical Tent B, helping runners who became sick after the race, when the first bomb hit.

“You heard boom and then a little bit of a pause and then boom and everyone was like, ‘What was that? That was not normal,’” she said.

The leader of her volunteer team directed her to move to Medical Tent A, where medical workers were assessing bomb victims in the field.

“When I got there there was a lot of blood and a lot of broken people, but it was surprisingly calmer than you’d think,” she said.

Solomon immediately saw a patient who was marked a priority.

“He was a little boy and his parents were holding him and his leg had just been tourniqueted and he was really bloody,” she said.

At nearby Medical Tent B, Gould said, about 10 minutes after the explosion, a stream of runners rushed down the road.

“They had no idea, they were just running because everybody else is running and we just told them just get as far away as you can,” Gould said.

After authorities evacuated Boylston Street, the area around Medical Tent B fell silent, she said, a peculiar hush just down the road from the epicenter of a brutal attack.

“It was eerie,” Gould said. “We ended up getting to the point where there was nobody around us it was just the volunteers and a couple runners here and there.”

Back at Medical Tent A, Solomon moved patients around in a wheelchair, pausing in between to wipe blood from the seat. She said she hopes to be a trauma nurse, and as tragic as the event was, it affirmed her career choice.

“I wish I had my nursing degree right now so I could actually put my hand on them and help them,” she said of her thoughts in the hours after the attack.

Yards away from Solomon, Hannah DeYoung had been handing out Gatorade to runners with members of her sorority, Sigma Sigma Sigma, all day. She was just past the finish line when the first bomb went off.

At first, she said, confused runners streamed past. Then came people who were closer to the blast site.

“After about a minute it was everybody who had been there when it happened,” DeYoung said. “So it was just people carrying I don’t even know, their children.”

DeYoung said she felt both fear and shock as she retreated and gathered with her sorority sisters. They later walked around the blast site, down Commonwealth Avenue to Massachusetts Avenue, to return to Northeastern.

The days since have been difficult, DeYoung said.

“We’re still trying to process, but we’re okay, we know that we’re okay and we’re going to be okay,” she said.

Alf Carroll, a senior biology major, was nearby handing out heat blankets with the Student Alumni Association (SAA) at Copley Square when the explosions shattered the jubilation on Boylston Street.

“Usually when you hear a bang or something, you think, oh, maybe there was a cannon or something that went off in front of the marathon,” he said. “But then you look back and you see all this smoke. And then everyone was kind of quiet. We looked at each other, and then the second bomb went off. Then people started to run.”

After the noise of the second bomb echoed down the road, it was clear that the explosions were not cannon fire, Carroll said.

“After the second bomb went off, people started to scream,” he said. “They were screaming ‘terrorists.’ You could hear them from down the street.”

Carroll and other people from the SAA ran into the South End. He called his family, and then thought of his job as an emergency room technician at Brigham and Women’s Hospital. He called and then went in.

The scene at the hospital was hectic as ambulances rushed marathon patients in for treatment.

“They brought down people from anesthesia, from orthopedics, literally every single team. There were probably fifteen different departments there,” Carroll said. “Everyone was just down helping and doing whatever they could do. There was definitely enough staff. We could have taken patients in every single room, and everyone would have had a full team. We were actually super prepared.”

The injuries varied, Carroll said, with some people suffering lacerations and others in critical condition.

“We definitely saw a few critical cases. We saw a couple life-threatening ones. But for the most part, most of the patients that came in suffered minor injuries,” he said. “People came in with debris that had gone through their clothing and cut them on their back or legs or something. A few people came in complaining that they had lost hearing or they had a headache.”

Despite all the pain and anguish, Carroll said the wounds were not the most trying part of Marathon Monday. The emotional effect was far more devastating.

“The injuries that we saw that were very critical from the explosions were not anything out of the ordinary. Having someone come in with their leg blown off is not out of the ordinary. We see stuff like that on a daily basis,” he said. “So it wasn’t the injuries that were shocking, but it was having something like that happen in your hometown, and just knowing that something like that is possible.”

Like others who witnessed the bombings and their aftermath firsthand, Carroll said, the extraordinary experience has provided him with a new perspective on the routine.

“Not only does it make you grateful for what we have, and how lucky we were that none of us were directly affected by what happened, it also snaps you back into reality,” he said. “You go through every day, and you’re kind of on autopilot. It’s unfortunate, but until something like this happens, you’re not even appreciating the life you have.”