Three spectators, including an eight-year-old boy, were killed when two pressure cooker bombs packed with nails, ball bearings and other shrapnel detonated at the 2013 Boston Marathon.
More than 260 people were also injured, with many losing limbs.
A police officer was killed and another seriously injured during the operation to catch the suspects, Dzhokhar and Tamerlan Tsarnaev.
Here we look at the victims.
The eight-year-old was standing with his family, cheering the runners as they completed the race. The prime position near the finish line put him in the path of one of the bombs.
He was the first person killed in the attacks to be publicly named. He was described as "kind, caring and loving" by staff at his former school.
His younger sister Jane, six, suffered a serious injury to her leg, while his mother, Denise, sustained a head injury and required emergency surgery.
In an emailed statement, Martin's father, Bill, wrote: "My dear son Martin has died from injuries sustained in the attack on Boston. My wife and daughter are both recovering from serious injuries. We thank our family and friends, those we know and those we have never met, for their thoughts and prayers."
The restaurant manager, 29, had gone to watch a friend complete the race.
Her mother, Patty, shaking with emotion, told reporters: "You couldn't ask for a better daughter... I can't believe this has happened. She was such a hard worker in everything she did."
She had lived with and cared for her grandmother for almost two years after a medical procedure, according to the New York Times.
"My daughter was the most lovable girl," her father told Yahoo. "She helped everybody, and I'm just so shocked right now."
The Chinese graduate student was 23.
Old friends and Chinese state media said she was from the north-eastern city of Shenyang. Her Linked-in profile said she had studied economics at Beijing Institute of Technology before coming to study statistics at Boston University.
She was an eager cook who liked to blog about her meals and share new recipes.
Boston University's website said she was one of three friends who watched the race near the finishing line.
One of her friends, named as Zhou Danling by Chinese TV, was wounded. A statement from the university said: "Our hearts and thoughts go out to the family and friends of both victims."
On the Thursday after the bombings, with Boston on high alert, Sean Collier, a police officer at the prestigious Massachusetts Institute of Technology (MIT), was found by colleagues dead in his car.
The 26-year-old had been shot multiple times, allegedly by Dzhokhar and Tamerlan Tsarnaev as they tried to evade arrest.
Police ordered Boston into lockdown, eventually tracking the suspects down, killing the elder brother and wounding the other.
Paying tribute to Mr Collier, Boston Police Commissioner Edward Davis said he believed "his murder led to our apprehension of these individuals".
MIT Police Chief John DiFava said in a statement that Mr Collier was "one of these guys who really looked at police work as a calling".
"He was born to be a police officer," he said. Students at the university said he had always made an effort to get to know them, so he "knew which students he was protecting every day when he came to work".
A second policeman, transport officer Richard Donohue, was shot in the thigh in a fire fight with the suspects when they were cornered later on that Thursday.