A recap of the Boston Marathon bombings, from explosion to arrest:
The race clock approaches the 4-hour, 10-minute mark as nonelite runners, their families and friends are focused on the finish line. A concussive sound echoes, then a white cloud with an orange fireball at its middle fills the north side of Boylston Street.
At about 2:50 p.m. the terrorist attack at the 117th Boston Marathon has begun.
Screams and car alarms continue when a second explosion occurrs about a block away, less than 10 seconds after the first.
Spectators, race volunteers, police and paramedics race toward the blasts. The injured, mainly spectators, fill the sidewalk behind race barricades after the explosions blow metal pellets and nails into victims' legs. Race tents become makeshift trauma units.
As social media sites start posting graphic images and video from the explosions, law enforcement sources tell NBC News that two small homemade bombs caused the blasts. Shortly after 4 p.m., Boston officials report two deaths and about 100 injuries, including some who had lost limbs.
At 6:10 p.m., President Barack Obama offers his condolences to the victims in a three-minute televised address. "We will find out who did this," he says. "We'll find out why they did this. Any responsible individuals, any responsible groups, will feel the full weight of justice."
Just before 9 p.m., Boston police announce the death toll has risen to three, including an 8-year-old boy. The number of injured grows to more than 170.
Police and federal agents are seen leaving a home in nearby Revere, Mass., but before noon, word comes from police sources that the search turned up nothing related to the investigation.
At 11:30 a.m., the president, in a news conference, calls the attack "an act of terror" but doesn't indicate whether it was carried out by foreign or domestic residents or groups.
More than 1,000 law enforcement officers start sorting through 2,000 tips. Investigators say in a news conference that they don't know who set off the bombs. FBI Special Agent in Charge Richard DesLauriers asks for the public's help. "Someone knows who did this," he says.
Forensic evidence recovered from the blast scene includes a circuit board, batteries and black powder. Police also recover black nylon, possibly from the bag used to carry the bombs to the marathon. A law enforcement source confirms that the bombs were made from pressure cookers packed with BBs and nails.
Officials identify two of those killed: restaurant manager Krystle Campbell, 29, of Medford, Mass., and the boy, Martin Richard, who was watching the race with his family. His mother and sister were critically injured in the blast.
Wednesday, April 17
Multiple media outlets, including The Associated Press and CNN, report that police had made an arrest, citing law enforcement sources. Hours later they are forced to retract the reports after investigators state officially that no arrest had been made and no suspect was in custody.
Later in the day, sources close to the investigation confirm that a department store surveillance camera caught images of two suspected bombers. Investigators were reported to be sharing the images with other law enforcement agencies.
Investigators say some debris, including a pressure cooker lid, was found on rooftops.
While police hunted for suspects, and loved ones grieved their loss, University of Massachusetts Dartmouth sophomore Dzhokhar Tsarnaev, who would later be identified as a bombing suspect, spends the day working out in a gym, going to a party and sleeping in his dorm room that night, according to the Boston Globe.
A student quoted by the paper said that Tsarnaev had been at a party Wednesday night that was attended by some of his soccer friends. "He was just relaxed," the female student said.
Thursday, April 18
Obama speaks at an 11 a.m. interfaith service at Boston's Cathedral of Holy Cross, telling a standing-room-only crowd and national television audience "I'm here today on behalf of the American people with a simple message -- every one of us has been touched by this attack on your beloved city. Every one of us stand with you."
Without an arrest three days after the blasts, investigators at about 6 p.m. publicly release images of the two alleged bombing suspects taken from a Lord & Taylor department store surveillance camera.
The man who became known as "Suspect No. 1" is wearing a black baseball cap, and "Suspect No. 2" has a white baseball cap on backwards and is carrying a black backpack. Authorities say they suspect the dark backpack was left in front of the Forum Restaurant on Boylston Street on Monday and then exploded -- the second blast.
"The nation is counting on those with information to come forward," the FBI's DesLauriers said.
About 10:20 p.m., shots are fired on the Massachusetts Institute of Technology campus in Cambridge, about 2 miles north of downtown Boston. Ten minutes later, an MIT campus police officer, Sean Collier, 26, is shot multiple times while still in his police cruiser during an apparent clash with the bombing suspects. He is later pronounced dead.
Two armed men carjack a Mercedes SUV in Cambridge a short time later. The robbery suspects hold the owner for an hour and release him unharmed at a gas station.
A source said the bombing suspects used the man's debit card to withdraw $800 from at least two ATMs.
Friday, April 19
Police locate and follow the Mercedes in Watertown, just west of Cambridge.
Around 1:30 a.m., explosives are thrown from the Mercedes at police, who exchange gunfire with the carjackers. A transit officer, Richard H. Donohue, 33, is seriously wounded.
On Laurel Street in Watertown, Mike Doucette, 27, is about to fall asleep while watching television about 2 a.m. when he hears gunfire and what he said sounded like bombs detonating outside.
Doucette said he looked outside and saw a man, later identified as Dzhokhar Tsarnaev, and a police officer, separated by about 30 feet, in a shootout. Doucette said he next saw another man walking up Laurel Street toward the police. That's when police shot the man identified as 26-year-old Tamerlan Tsarnaev, Dzhokhar Tsarnaev's older brother.
Less than a minute later, the Mercedes speeds up the street toward the police and the older brother. The police are able to get out of the way, but the SUV speeds over Tamerlan Tsarnaev, Doucette said.
About 4:30 a.m., police tell residents in eastern Watertown to stay in their homes, and announce that the carjackers and the bombing suspects are the same two men. Police release a photo from a cash machine camera that shows 19-year-old Dzhokhar Tsarnaev wearing a gray hooded sweatshirt.
All Boston-area mass transit is shut down at 5:50 a.m. Less than an hour later, The Associated Press reports that the bombing suspects are of Chechnyan ancestry and lived in the United States for at least a year. Dzhokhar Tsarnaev is identified as the surviving suspect.
Police recover the Mercedes, and, with daylight, begin searching for Dzhokhar Tsarnaev in Watertown, including going door-to-door through neighborhoods.
Gunfire is heard in a Watertown neighborhood being searched by police at about the time the governor concludes his remarks.
Following a tip from a homeowner who called 911 after finding blood on a boat parked in his yard, police, aided by a state helicopter equipped with thermal imaging detection, find the injured Tsarnaev under a tarp on the boat.
About 8:45 p.m., he is captured taken to a hospital.
Onlookers break into spontaneous applause. As police pull their vehicles from the scene, some residents take to the streets to cheer.
Saturday, April 21
The FBI take two men into custody at the Hidden Brook apartment complex in New Bedford for questioning in connection with the marathon bombing investigation, a federal source says. They are questioned and released.
The governor, speaking outside Fenway Park where the Boston Red Sox played the Kansas City Royals, says Tsarnaev's condition is serious but stable. Investigators haven't been able to question him because of his injuries.
"He's not yet able to speak so there'll be some time in the recovery," Patrick says. "I, and I think most of the law enforcement professionals, are hoping for a host of reasons that the suspect survives, because we have a million questions, and those questions need to be answered."
Ther governor meets with some of the injured, telling them, "You're going to get better and we're going to get better. We'll rebound and we'll rebuild."
With Kevin Deutsch, Mackenzie Issler, Matthew Chayes and Nicole Fuller