It began on Monday with two bombs, about 10 seconds apart, just shy of the finish line of the Boston Marathon, blasts that sounded the start of a new race - to identify and find those responsible. This is how that race unfolded.
Monday, April 15
Just before 3pm, an explosion shatters the cheers near the finish line of one of the world's most significant marathons. More than 17,000 runners already had crossed the finish line, but thousands more still were on the way. Seconds later, another explosion shatters windows and bodies. Sirens and screams erupt as rescuers scramble and the crowd panics.
"They just started bringing people in with no limbs," runner Tim Davey said of his view from inside a medical tent that had been set up to care for fatigued runners.
The blasts killed three - 8-year-old Martin Richard, 29-year-old Krystle Campbell and 23-year-old Lu Lingzi, a graduate student from China - and injured more than 180, but it would be hours before the chaos cleared enough to give authorities a true sense of the casualties.
A citywide shutdown that will become nearly complete by the end of the week begins. A no-fly zone is created over the bombing sites, major sporting events are canceled, people are urged to stay indoors, police officers with machine guns patrol hospitals.
Is it terrorism? Americans are eager for answers, but when President Barack Obama addresses the nation, he stops short of saying that. "We will find out who did this. We'll find out why they did this," he says.
Knowing that thousands of smartphones and cameras were in the crowd, authorities put out the call for pictures, videos and tips.
Tuesday, April 16
The day begins with a city - and a nation - on edge and without answers. No suspects. No motives. No claims of responsibility.
Calling the bombings "a heinous and cowardly act," Obama says they are being investigated as an act of terror, but authorities still don't know who is responsible.
A picture of the bombs begins to emerge. Based on debris at the site, investigators determine they were crudely fashioned from kitchen pressure cookers packed with explosives, nails and ball bearings. They were hidden in black backpacks and left on the ground.
Pictures of the victims emerge, too. There is Martin Richard, smiling and holding a sign that calls for peace and reads, "No more hurting people". And there is 27-year-old Jeff Bauman Jr, being pushed in a wheelchair from the scene, both legs blown off below the knees.
It would be another two days before pictures of the suspects, 19-year-old Dzhokhar A Tsarnaev and his brother, 26-year-old Tamerlan Tsarnaev, would emerge. But already the younger of the two appears nervous. The owner of an auto body shop near the brothers' home later recalled a visit from Dzhokhar on Tuesday.
Gilberto Junior said the usually easygoing teen often stopped by to talk cars and soccer. But on Tuesday, he was biting his nails and trembling. The mechanic told Dzhokhar he hadn't had a chance to work on a Mercedes the teen had dropped off for bumper work. "I don't care. I don't care. I need the car right now," Gilberto says Dzhokhar told him.
Wednesday, April 17
Investigators go through thousands of tips, scour the rooftops and roads around the blast site and use sophisticated software to sift through mountains of images and video for patterns or unusual behavior.
Investigators discover department store surveillance footage shot near the site of the bombs that shows a man dropping off a bag believed to contain one of the bombs. But officials say they still don't know the man's name.
Boston remains under a heavy security presence, with police officers stationed on street corners across the city.
Thursday, April 18
Obama and other dignitaries attend an interfaith service. "You will run again!" Obama says.
Using facial recognition technology and a painstaking frame-by-frame search, investigators have narrowed their search to images of two young men, Suspect No 1 and Suspect No 2.
At 5:10pm, investigators reveal the photos and video of the two men, in hopes they will be identified or reveal themselves. But it comes with risk. The men could lash out with more violence. Within moments, the FBI's website is overwhelmed.
"We consider them to be armed and extremely dangerous," FBI agent Richard DesLauriers says.
At 10:20pm, shots are heard on the campus of the Massachusetts Institute of Technology, across the Charles River from Boston in Cambridge. Ten minutes later, a 26-year-old campus police officer is found shot multiple times in his car and pronounced dead. He had been responding to a report of a disturbance.
Shortly after, two armed men carjack a Mercedes SUV in Cambridge. They hold the driver for half an hour, then release him unharmed. That man runs into a gas station and calls the police. It convinces police they are dealing with the bombing suspects.
The search for the Mercedes leads to a chase that ends in Watertown. Residents describe war zone-like scenes, with the suspects hurling explosive devices from the car and exchanging gunfire with police. The men had collected pipe bombs, grenades and improvised explosive devices. A transit police officer is shot and critically wounded.
In the gunfire, Tamerlan Tsarnaev is shot. Dzhokhar Tsarnaev escapes in a stolen vehicle, running over his wounded brother as he flees. In his wake - 200 spent shells. Tamerlan Tsarnaev dies shortly after at a Boston hospital from multiple gunshot wounds and a possible blast injury. Meanwhile, at some point his brother abandons his car and flees on foot.
Friday, April 19
Gunshots and explosions are heard in Watertown around 1am police teams, sharpshooters and FBI agents descend on an area stretching from Watertown to Cambridge. Police helicopters buzz overhead and armoured vehicles rumble through the streets. By 4:30am, residents of eastern Watertown are told to stay in their homes.
An hour later, the lockdown is extended across Boston, affecting more than 1 million people. Open your door only for uniformed officers, they are told. Mass transit is shut down. Businesses are told not to open.
"We believe this man to be a terrorist," Boston police commissioner Ed Davis says. "We believe this to be a man who's come here to kill people."
Investigators begin a methodical, door-to-door sweep of Watertown. By midmorning, the University of Massachusetts Dartmouth evacuates its campus after confirming that Dzhokhar Tsarnaev was registered there. Around midday, the suspects' uncle, Ruslan Tsarni, pleads on television: "Dzhokhar, if you are alive, turn yourself in and ask for forgiveness."
Dzhokhar Tsarnaev had somehow evaded an army. As night fell, authorities start scaling back the hunt. Mass transit is allowed to resume, and people are told they can leave their homes.
But just as that order is lifted, there is a break. A man in Watertown sees blood on a boat parked in a yard. When he pulls back the tarp, he sees a man covered in blood and calls police. When authorities arrive, they try to talk the suspect - already weakened by a gunshot wound received some 20 hours earlier - into getting out of the boat.
Police say the 19-year-old suspect exchanged gunfire with law enforcement for an hour while hiding in the boat before being captured.
Just before 9pm, Boston police take to Twitter: "CAPTURED!!! The hunt is over. The search is done. The terror is over. And justice has won. Suspect in custody."
People pour into the streets. Church bells ring. American flags are waved. A city erupts again.