On a sunny Monday afternoon in April 2013, spectators lined the streets of Boston to cheer on runners in one of America’s most famous competitions, the Boston Marathon. But just before 3 p.m. the festivities suddenly turned into chaos and horror, as two homemade bombs, set 12 seconds apart, detonated near the finish line. The blasts killed three people and injured another 281, including 12 who suffered such severe injuries that they would have to undergo amputations.

In the wake of the terrorist attack, the Boston area went on lockdown out of fear that the perpetrators would strike again. Federal, state and local law enforcement officials rushed into action, launching an all-hands-on-deck effort to catch them. Four-and-a-half days later, it was all over. One of the two terrorists, a 26-year-old immigrant of Chechen descent and once-promising amateur boxer named Tamerlan Tsarnaev, was dead of injuries suffered in a shootout with police. His 19-year-old brother and collaborator Dzhokhar Tsarnaev, was captured while hiding in a backyard in a Boston suburb.

Below are key moments from that tragic day and the intense manhunt that followed.

Monday, April 15, 2013: The Bombings

Around 2 p.m., Tamerlan Tsarnaev walked down Boylston Street in Boston, carrying a backpack, according to a federal grand jury indictment of his brother Dzhokhar. Inside the pack was an improvised explosive device, or IED, constructed from a pressure cooker, according to instructions downloaded from the Internet. He allegedly placed the remote-control bomb amid a crowd of spectators. About the same time, Tamerlan’s brother Dzhokhar was a short distance away on the same street, where he planted a similar device. The two brothers then slipped away and a few minutes later had a short cell phone conversation. 

At 2:49 p.m., the first of the Tsarnaevs’ bombs detonated at 671 Boylston Street. Twelve seconds later, a second bomb went off about 180 yards away at 755 Boylston. Police, fire and emergency medical responders rushed into action, providing care to critically injured spectators and rushing them to area hospitals. Remarkably, every patient who got to a hospital survived.

About 40 minutes after the attack, an assortment of local, state and federal officials set up a temporary unified command center at the Westin Hotel in Boston, where they analyzed intelligence and tried to figure out whether there is a threat of additional attacks, and began collecting surveillance-camera video from businesses near the bombing site. 

Ultimately, more than 20 law enforcement agencies with more than 1,000 investigators joined the investigation, according to a Boston Globe account. As the night wore on, federal agents took the lead in the investigation, questioning witnesses late into the night.

Tuesday, April 16, 2013: Investigation Intensifies

By early morning, investigators had combed a 15-block zone in the vicinity of the attack, carefully stepping over broken glass and bloodstains and creating a massive grid in which they searched for potential evidence. They found fragments of the bombs, including the lid from one pressure cooker, which had been thrown by the blast onto a nearby roof.

As the day wore on, other investigators scrutinized thousands of photographs and videos that had been gathered from businesses, Boston residents and tourists.

Meanwhile, the bombers blended back into their everyday existence. Dzhokhar Tsarnaev stopped by a body shop in Somerville, a Boston suburb, to check on a car that he had left there, posted a few messages on Twitter, including some lyrics to an Eminem song, and went to a fitness center, according to the Globe.

Wednesday, April 17, 2013: Surveillance Video Images Found

After studying video through the night, investigators began to focus on two figures who showed up in footage taken just minutes before the explosions. As later described in an FBI agent’s affidavit, one surveillance video depicted the men turning onto Boylston Street, carrying large backpacks. “Bomber One,” eventually identified as Tamerlan Tsarnaev, was wearing a dark-colored baseball cap, sunglasses, and a dark coat and tan pants. “Bomber Two,” later identified as Dzhokhar Tsarnaev, was in a gray hooded sweatshirt and dark jacket and pants, with a white baseball cap turned backwards. 

In other footage, Bomber Two could be seen slipping his backpack onto the ground, and then making a phone call. Seconds after the first explosion, as race spectators fled in panic, Bomber Two appeared strangely calm as he walked away, without his backpack.

But figuring out the identities of the suspects in grainy surveillance video images proved to be a challenge. Eventually, video obtained from a local business owner was sharp enough to identify Dzhokhar more clearly.

Thursday, April 18, 2013: MIT Police Officer Shot and Killed

As President Barack Obama visited Boston that morning to speak at a prayer service and meet with victims, law enforcement officials pondered whether they should go public with the video images of the bombing suspects, according to the Globe. Finally, at a press conference at around 5 p.m., the FBI publicly released photos and descriptions of the two young men, and asked the public for help in identifying and finding them.

The Tsarnaevs knew that the net was tightening. At 8:45 a.m., a few hours after TV stations began showing a photo of Dzhokhar Tsarnaev that identified him as a suspect, he sent a text message to one of his college classmates, inviting him to go to his dormitory room and help himself to any of Dzhokhar’s possessions that he wanted, according to the indictment.

At about 10 p.m. that evening, the Tsarnaevs got into a Honda Civic and drove to the MIT campus. They allegedly were armed with a Ruger 9mm semiautomatic handgun, a machete, a hunting knife and five more homemade bombs, according to the indictment.

About a half-hour later, Sean Collier, an officer with Massachusetts Institute of Technology’s police department, was sitting in his police car on the MIT campus in Cambridge when he was shot to death by one of the Tsarnaevs. The brothers tried unsuccessfully to steal Collier’s service weapon from his body, but were thwarted by the locking system in Collier’s holster, as CBS News later reported.

Friday, April 19, 2013: Kidnap Victim Alerts Police

At about 11 p.m., the Tsarnaevs carjacked a Mercedes SUV and kidnapped its owner. They drove the victim to a bank ATM machine and used his card to withdraw $800. The brothers allegedly indicated that they planned to drive to Manhattan in New York City. 

Shortly after midnight, the Tsarnaev’s captive escaped from the carjacked Mercedes and called 911, according to the indictment. In Watertown, a suburb of Boston, local police caught up with the Tsarnaevs, who shot at them and used four of their improvised explosive devices, including another pressure-cooker bomb. 

Tsarnaev Killed, Dzhokhar Goes Into Hiding

After a 7-minute firefight, three police officers managed to tackle a wounded Tamerlan Tsarnaev, who struggled with them as they tried to handcuff him. Dzhokhar Tsarnaev got in the Mercedes and tried to run over the officers, but instead hit his own brother, seriously injuring him and contributing to Tamerlan’s death shortly afterward in a Boston hospital. A Massachusetts Bay Transit Authority Police officer who responded to the scene was wounded, possibly accidentally by police fire, but survived.

Dzhokhar Tsarnaev fled in the Mercedes, but didn’t go too far. He abandoned the car on a street in Watertown, smashed his two cell phones to make it more difficult to track him, and hid inside a boat stored in the backyard of a house, according to the indictment. While he lay in hiding, he wrote a message on the inside wall of the boat, saying that the terror attack was in response to the U.S. wars in Muslim countries.

Dzhokhar Tsarnaev Found, Arrested

At 6:42 p.m., Watertown police received a call from a resident who had spotted Dzhokhar Tsarnaev. Within moments, more than 100 officers from various departments had converged on the scene. In the pandemonium, an officer fired on the boat without authorization, and others—mistakenly thinking the shot came from the suspect—began shooting as well. After the brief fusillade, infrared imaging from a police copter confirmed that Dzhokhar Tsarnaev was still alive, and after a two-hour standoff, the wounded suspect emerged and was arrested. 

Aftermath of the Bombings

On May 15, 2015, Dzhohkar Tsarnaev, the surviving bomber, was convicted by a federal jury in Boston on 30 charges related to the bombing and the killing of officer Collier, and was sentenced to death by lethal injection on six of the counts. At his trial, he apologized to survivors and family members of the victims. He was sent to the federal supermax prison in Florence Colorado, known as “The Alcatraz of the Rockies,” where he was incarcerated with terrorists such as Unabomber Ted Kaczynski and airline “shoe bomber” Richard Reid. Tsarnaev’s death sentence was later overturned by a federal appeals court, on grounds of possible jury exposure to media coverage and exclusion of evidence, but in March 2022 it was reinstated by the U.S. Supreme Court.

Boston, meanwhile, bounced back from the trauma. 

“We will forever remember and honor those who lost their lives and were affected by those senseless acts of violence on our City,” then-mayor Martin Walsh said after Dzhokhar Tsarnaev’s sentencing. “Today, more than ever, we know that Boston is a city of hope, strength and resilience, that can overcome any challenge.