After the bombs went off at the 2013 Boston Marathon -- a dark plume of smoke rising above the finish line -- the city descended into chaos, with ambulances rushing, people crying on the streets and images of bloodied victims broadcasting to a world watching in horror. Two years later, Boston will celebrate its marathon Monday, the runners determined to honor those who suffered in the terrible attack.

Sean Astin will be one of 30,000 participants pounding the pavement in the 2015 Boston Marathon. The father of three will be fighting through the 26.2-mile course for the foundation in memory of 8-year-old Martin Richard, who lost his life in the 2013 marathon bombing. Astin, an actor known for portraying Daniel “Rudy” Ruettiger, Sam Gamgees from "Lord of the Rings" and Mikey from the "The Goonies," has raised more than $16,000 and helped get the word out for  Team MR8, the group of runners participating in the marathon for the organization founded in Richard's memory.

“I have three daughters, my youngest daughter is nine, she would have been seven during that time,” Astin said. “Anyone who saw -- the whole country watched -- was moved.”

Marathon Monday has always been a special day for Boston -- a celebration of spring, a day off work, a communal party -- but since 2013’s bombing, it’s taken on a weightier significance. The field of the 2015 marathon is brimming with runners determined to honor the dead and those who survived. Ask them about the marathon and they’ll bring up charities, victims and Boston’s resilience. They’ll remember where they were on April 15, 2013, when the pressure-cooker bombs went off at the finish line. They’ll talk of community. They’ll say they want to be a part of something bigger, a day that honors those affected by a tragedy and celebrates an entire city.

April 15, 2013

On a sunny day with thousands of people cheering, the bombs went off shortly before 3 p.m near the finish line. Three people -- Richards, 8, Krystle Campbell, 29, and Lingzi Lu, 23, -- were killed, while others were injured, 264 receiving hospital treatment. On Friday of that week, MIT police officer Sean Collier, 27, was shot and killed by brothers Tamerlan and Dzhokhar Tsarnaev. Tamerlan would later die in a shootout with police, his brother contributing to the death by accidently hitting him with a car while fleeing. Dzhokhar was later caught and convicted on April 8 of 30 charges, including deadly use of a weapon of mass destruction and conspiracy. He awaits sentencing, facing the death penalty or life in prison.

The day of the bombing, word of the attack took time to filter through the crowds lining the course. Phone calls and text messages struggled to get out, as people across the city and country waited to hear from loved ones. Emergency messages played and replayed on television stations.

The immediate task was “find out if our friends were okay, if our friends were near,” said Miguel Perez-Luna, a 21-year-old senior at Harvard, who will be running the 2015 race. Later, the horrifying details about the attack and victims would emerge.

Maria Kalina, a 20-year-old junior at Harvard, had gone to the marathon for years, watching her dad finish the race eight times from the finish line. A midterm test was all that kept Kalina and her brother away from the watching the race in 2013.  “I definitely was left with some scars and was traumatized by the whole experience,” Kalina said.

After the attack, with the bombers still on the run, the city was largely shut down. Areas usually bustling, like Fenway-Park-adjacent Kenmore Square, were eerily empty. But Boston’s response was swift, and by 2014 the marathon had become a symbol for the town's strength.

"The city unites as one. Everyone now is there for the same reason, everyone's there to support the city," Kalina said.

Running for a reason

More than 80 percent of the runners in the 2015 race ran a qualifying time that allowed them to enter the field. A large portion of the other invited runners are representing charities, including those that benefit the victims of 2013, like the Martin Richard Foundation.

Casey McDonagh, a University of Massachusetts employee and alumna, is representing the UMass foundation in honor of its alumna Campbell, who died in the 2013 attack. McDonagh, 33, lives in the Dorchester neighborhood of Boston and struggled through a record-breaking winter -- which saw 110.6 inches of snow -- to prepare for the race. “There’d be times I’d be climbing over snow banks,” she said. “I was running back and forth on the same street or [spending] hours on the treadmill.”

But she kept going. “I always just remember why I’m running,” McDonagh said. “I wanted to try and do something that would turn something good out of an awful tragedy.”

Kalina had to push through training runs during blizzards to get ready for the 2015 marathon. Her routes would take her past landmarks from the 2013 attacks, including the gas station near MIT’s campus where the Tsarnaev brothers carjacked a Mercedes Benz to stay on the run.

“When I run past the Shell station where everything happened that Thursday night, I find myself running faster out of anger,” Kalina said. “During training runs you have to tell yourself why you’re doing it.”

Maggie Crowley -- a 20-year-old sophomore at Boston College who will also run Monday for MR8 -- has seen people set up drink and snack stops alongside training courses to help the 2015 marathoners.

“The togetherness in Boston -- I never realized how big of a deal the marathon was until I [became] a part of it,” she said.

Recently a pre-marathon run through Dorchester for the Martin Richard Foundation had neighbors outside yelling in support. The group caused some unforeseen congestion.

“Traffic was stopped in Dorchester at 7 o’clock,” Crowley said. “Instead of honking, they had their heads out of windows cheering.”

“Go out there and run, like we’ve always done.”

About one million spectators line the course of the Boston Marathon every year. The crowd’s support turns every corner with the runners.

It’s the world’s oldest annually-contested marathon. The attack in 2013 only seemed to strengthen the marathon's popularity, turning the day into both a celebration and a considered remembrance of the tragedy. Boston will “go out there and run, like we’ve always done,” Perez-Luna said.

The race weaves in a serpentine line from the small town of Hopkinton, into Boston, down Commonwealth Avenue, up the infamous Heartbreak Hill at about the 20.5 mile-marker and finally concludes at the world-famous finish line on Boylston Street in Copley Square. A back-breaking, knee-aching amount of training and -- for the charity runners -- loads of fundraising culminates in those 26.2 miles.

Waiting at the end will be a city celebrating its day – something uniquely its own. People will cheer, backs will be patted, medals doled out. The victims of 2013 will be honored, thousands of dollars raised in their names. Thirty thousand people will run the course and a city will remind itself how unified it can be.

“Long-distance runners have the ability to go into an altered state,” Astin said about running 26. 2 miles. “But at certain moments, usually when it gets really hard … you can find certain things to hold on to.”

He’ll hold on to his wife and children, their names written on the back of his shirt. But he said he’ll also hold on to Martin Richard, the Richard family and the idea that the race is for all those affected in 2013. He’ll think about the city and the runners around him.

“It’s Boston’s day,” he said.