— Just before 3 p.m., an explosion shatters the cheers on Boylston Street near the finish line of one of Boston's largest and most cherished events. More than 17,000 runners already had crossed the finish line, but thousands more still were headed for the site of the bombing. Ten seconds later, a second 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 of Richmond, Va., said of his view from inside a medical tent that had been set up to care for fatigued runners.
— The blasts killed three people — 8-year-old Martin Richard, of Boston; 29-year-old Krystle Campbell, of Medford; and 23-year-old Lu Lingzi, a Boston University graduate student from China — and injured more than 180 others, but it would be hours before the chaos cleared enough to give authorities a true sense of the casualties. Or even where the bombs had been hidden. If they had been hidden.
— A citywide shutdown that will become nearly complete by the end of the week begins by early evening. A no-fly zone is created over the bombing sites, major sporting events are canceled, people are urged to stay indoors, SWAT team members with machine guns patrol hospitals. And the world takes notice, beefing up security at nuclear plants, public transit systems and anywhere crowds gather.
— Is it terrorism? Americans are eager for answers, but when President Barack Obama addresses the nation three hours after the explosion, he stops short of that. "We will find out who did this. We'll find out why they did this," Obama said in his brief statement. "Any responsible individuals, any responsible groups, will feel the full weight of justice."
— Knowing that thousands of smartphones and cameras were in the crowd, by nightfall authorities officially tap the power of crowd sourcing and 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. An apartment in nearby Revere was searched overnight, but no details emerge. Copley Plaza — the typically bustling site of the bombings — was blocked to vehicles and pedestrians.
— By noon, Obama inches the nation forward but only barely. Calling the bombings "a heinous and cowardly act," he says they are being investigated as an act of terrorism, but authorities still don't know who is responsible. Later in the day, Homeland Security Secretary Janet Napolitano says the bombings don't appear to be part of a large plot, but security on public transit nationwide — and around the world — remains high.
— A picture of the bombs begins to emerge. Based on debris at the site, investigators determine the bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings. And they were hidden in black backpacks and left on the ground.
— Pictures of the victims emerge, too. Photos flood social media. There is 8-year-old 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 of the explosions, bloodied with 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' Cambridge 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," Junior says Dzhokhar told him.
WEDNESDAY, APRIL 17
— Investigators plow 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. Obama signs an emergency declaration to send federal aid to Massachusetts.
— Amid conflicting — and ultimately false — reports of a break in the case, 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. National Guardsmen have set up tents on the Boston Common and stationed tactical vehicles.
THURSDAY, APRIL 18
— Obama and other dignitaries attend an interfaith service at the Cathedral of the Holy Cross. "You will run again!" Obama tells the city.
— 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. But officials still don't know who they are. And as of 1 p.m., they won't publicly describe them.
— At 5:10 p.m., investigators reveal the photos and video of the two men, a tactic intended to apply pressure in hopes the men will be identified or reveal themselves. But it comes with risk. The men could reveal themselves by lashing out with more violence. Within moments, the images trigger a flood of responses that overwhelm the FBI's website.
"We consider them to be armed and extremely dangerous," FBI Agent Richard DesLauriers said.
— About five hours later, something happens. It's not clear what, but something snaps and the city of Boston seems to spin violently out of control for nearly 24 hours.
— At 10:20 p.m., 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 MIT campus police officer, Sean Collier, is found shot multiple times in his car and pronounced dead.
— 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 police. Whatever the carjackers had told him during his time with them convinces police they are dealing with the bombing suspects.
— The search for the Mercedes leads to a chase that ends in Watertown. Residents there describe war zone-like scenes, with the suspects hurling explosive devices from the car and exchanging gunfire with police. The men were prepared. They had collected pipe bombs, grenades and improvised explosive devices, police say. A transit police officer, 33-year-old Richard Donohue, is shot and critically wounded.
— In the course of the gunfire, 26-year-old Tamerlan Tsarnaev is shot. His brother, Dzhokhar Tsarnaev, escapes in a stolen vehicle, running over his wounded brother as he flees, according to police. In his wake are 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 1 a.m. Police SWAT teams, sharpshooters and FBI agents descend on an area stretching from Watertown to Cambridge, surrounding buildings. Police helicopters buzz overhead and armored vehicles rumble through the streets. Trains are searched. And by 4:30 a.m., 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, including Amtrak trains to New York. Businesses are told not to open. It is a city paralyzed. Signs up and down the highways leading into Boston warn to avoid the city.
"We believe this man to be a terrorist," Boston Police Commissioner Ed Davis said. "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 closes and evacuates its campus after confirming that Dzhokhar Tsarnaev was registered there. Around midday, the suspects' uncle, Ruslan Tsarni, of Maryland, 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.
— He doesn't. Police say the 19-year-old suspect exchanges gunfire with law enforcement for an hour while holed up in the boat before being captured.
— Just before 9 p.m., 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.