After twin blasts shook Boston -- killing three and wounding more than 260 others -- investigators sprung into action looking for those responsible.
New details emerged about that probe Wednesday, when three friends of surviving bombing suspect Dzhokhar Tsarnaev were charged in relation to the case. FBI affidavits from criminal complaints tied to their arrests help explain more about what happened April 15 and in the subsequent days.
Here is a chronological account of developments in the case and subsequent investigation, as spelled out in court documents and other official statements, including a federal complaint released Wednesday. The names below -- including those of suspects who have not been convicted of any crime -- reflect authorities' official explanation as to what they believe occurred.
Monday, April 15
Morning: The Boston Marathon kicks off from Hopkinton, Massachusetts, with runners and wheelchair competitors over the next few hours heading toward the state capital. Some 23,000 people participate in the race, with many times more people cheering them along the 26.2-mile route.
2:38 p.m. Cameras show two men -- later identified by authorities as Tamerlan and Dzhokhar Tsarnaev -- turning onto Boylston Street, the road where the marathon's finish line is located.
2:42 p.m.: Tamerlan Tsarnaev detaches himself from the crowd and begins walking east toward the marathon finish line. He walks past the Forum restaurant, while carrying a knapsack, toward where the first blast will soon occur.
2:45 p.m.: His brother Dzhokhar Tsarnaev begins walking toward the finish line, with his right thumb seemingly hooked under his backpack strap and a phone in his left hand. He stops in front of the Forum restaurant, standing by a metal barrier alongside marathon spectators.
He then seemingly slips his knapsack to the ground, with a photo later showing the bag at his feet. Dzhokhar Tsarnaev remains here for about four minutes, looking at his cell phone and once seeming to take a picture with it.
2:49 p.m.: Dzhokhar Tsarnaev lifts the phone to his head and speaks for about 18 seconds.
He finishes the call, and the first explosion goes off within seconds. Most everyone along Boylston turns toward the finish line yet Dzhokhar Tsarnaev appears calm. He looks to the east, then starts moving west -- away from the finish line, and not carrying the knapsack he'd once had.
About 10 seconds later, there's an explosion where Dzhokhar Tsarnaev had put the bag -- about a block from the first blast.
Wednesday, April 17
Afternoon: Dias Kadyrbayev -- a friend of Dzhokhar Tsarnaev since the two enrolled together at the University of Massachusetts-Dartmouth the same time in fall 2011 -- drives to Tsarnaev's dormitory room at Pine Dale Hall on the southern Massachusetts school's campus, he tells investigators as recounted in an FBI affidavit. He texts his friend to come down to meet him.
Kadyrbayev notices that Dzhokhar Tsarnaev has cut his hair short. The two chat while Kadyrbayev smokes a cigarette, then Tsarnaev goes back to his dorm room.
Evening: Azamat Tazhayakov -- Kadyrbayev's roommate at a New Bedford apartment -- later tells investigators that he's with Dzhokhar Tsarnaev at the apartment until about midnight. (It is not clear if Kadyrbayev was there at the time.)
Thursday, April 18
Afternoon: Azamat Tazhayakov attends a class at the University of Massachusetts-Dartmouth, after which Dzhokhar Tsarnaev drives him home to his New Bedford apartment.
5 p.m. ET: The FBI releases pictures of two male suspects being sought in connection with the Boston Marathon bombings.
5 p.m. to 6 p.m.: As he is driving home from Boston, Kadyrbayev says that he talked on the phone with Robel Phillipos -- who also enrolled at the southern Massachusetts university at the same time as him, Tazhayakov and Dzhokhar Tsarnaev.
Phillipos tells Kadyrbayev to put the news on when he gets home, because one of the Marathon bombing suspects looks familiar.
Kadyrbayev later turns on the TV and notices one of the suspects resembles Dzhokhar Tsarnaev, according to an FBI affidavit. Kadyrbayev's lawyer, Robert Stahl, later disputes that assertion -- saying his client didn't immediately recognize Dzhokhar Tsarnaev as a possible bombing suspect and did not know that his friend allegedly was involved in the attack, saying "his first inkling came much later."
Kadyrbayev texts Dzhokhar Tsarnaev to tell him he looks like one of the bombing suspects. His friend's response includes "LOL" -- Internet slang meaning laughing or laugh out loud -- and other things Kadyrbayev said he interprets as jokes like, "You better not text me" and "Come to my room and take whatever you want."
6 p.m. onward: Kadyrbayev later tells investigators that, between 6 p.m. and 7 p.m., he, Tazhayakov and Phillipos meet on the University of Massachusetts-Dartmouth campus and go to Tsarnaev's dorm room.
Tazhayakov claims that it was around 9 p.m. when Kadyrbayev texted him alerting him to the "news" and their friend's possible involvement in the Boston blasts. The two meet at the New Bedford apartment they share, at which time Kadyrbayev shows Tazhayakov photos broadcast by CNN of a suspect he believes is their friend Dzhokhar Tsarnaev, according to Tazhayakov's account to investigators.
It's at this time, Tazhayakov later recalled, that Kadyrbayev texts Phillipos so they can all meet at Dzhokhar Tsarnaev's dorm room.
Dzhokhar Tsarnaev's roommate lets all three young men into the room, telling them that Tsarnaev had left a few hours earlier. Just before they go in, Tazhayakov says that Kadyrbayev showed them a text message from Tsarnaev saying, "I'm about to leave if you need something in my room take it."
"When Tazhayakov learned of the message, he believed he would never see Tsarnaev alive again," a federal affidavit states.
While watching a movie, the three -- Kadyrbayev, Tazhayakov and Phillipos -- spot a backpack with fireworks that have been opened and emptied of powder. Phillipos tells investigators that he noticed about seven tubular fireworks, each between six to eight inches long, in the backpack.
According to Tazhayakov's account in an FBI affidavit, Kadyrbayev also finds a jar of Vaseline that he thought Dzhokhar Tsarnaev used "to make bombs."
The same affidavit states that, at that time, Kadyrbayev knew Dzhokhar Tsarnaev was involved in the Marathon bombing -- an assertion that his lawyer denies.
The young men take take the backpack out of the dorm room, as well as Tsarnaev's laptop.
Around 10 p.m.: Kadyrbayev, Tazhayakov and Phillipos take the backpack and computer to the New Bedford, Massachusetts, apartment shared by the first two young men. There, they watch news reports that broadcast photos of the man who would later be identified as their mutual friend, Dzhokhar Tsarnaev.
Phillipos says later that Kadyrbayev and Tazhayakov both "started to freak out, because it became clear from a CNN report that we were watching that (Dzhokhar Tsarnaev) was one of the Boston Marathon bombers." He adds he did not understand much of what the roommates, Kadyrbayev and Tazhayakov, said afterward because they talked mostly in Russian.
According to Kadyrbayev, the three collectively decide to throw away the backpack and fireworks because they don't want Dzhokhar Tsarnaev to get in trouble. Kadyrbayev puts them in a black trash bag, which he tosses in a dumpster outside his apartment.
In federal documents, Tazhayakov gives different timing, as to when the backpack was discarded. He said it happened around 6 a.m. the next day, after he, Kadyrbayev and Phillipos see news reports that identify their friend, Dzhokhar Tsarnaev, as one of two Boston attack suspects. He said that Kadyrbayev decided -- and Tazhayakov agreed -- that they should toss the backpack.
Phillipos said Kadyrbayev asked him about 11 p.m., and he responded, "Do what you have to do." By the time he woke up after a two-hour nap, Phillipos recalled, the backpack was gone. After waking, Phillipos stays up until 4 a.m. Friday watching television with Tazhayakov.
11 p.m. Massachusetts Institute of Technology police officer Sean Collier, 26, is shot dead on the school's Cambridge, Massachusetts, campus -- a killing authorities later link to the Tsarnaevs.
Friday, April 19
Early hours: Police say the two suspects hijack a car at gunpoint in Cambridge, Massachusetts, taking the driver as a hostage. One suspect tells the driver they are the Boston Marathon bombers, and the suspects talk openly about heading to New York.
Eventually, though, the driver is able to escape his captors by running from them into a gas station convenience store.
Thanks in large part to information from the hostage, authorities track down the suspects. A chase ensues, during which the suspects toss explosives and exchange gunfire with police.
Tamerlan Tsarnaev dies after the gunfight, while his brother eludes authorities.
Daytime: Boston and surrounding communities are put on lockdown -- with schools closed, public transit halted and people ordered off the streets -- as authorities hunt for the surviving suspect.
Sometime that afternoon, some 60 miles south in New Bedford, a garbage truck clears the dumpster outside Kadyrbayev and Tazhayakov's apartment. The dumpster had contained Tsarnaev's tossed backpack.
6 p.m. to 9 p.m.: After the lockdown is lifted, a resident goes outside to check on his boat parked in the backyard and notices blood inside. His tip leads to a large-scale law enforcement effort that culminates with the capture of Dzhokhar Tsarnaev.
When the suspect is found, he has visible injuries including apparent gunshot wounds to his head, neck and legs and hand. His wounds are treated, and he is transported by ambulance to Boston's Beth Israel Deaconness Medical Center.
April 19 and 20
Investigators interview Dias Kadyrbayev over parts of two days. He discusses his relationship with Dzhokar Tsarnaev, whom he met when they both enrolled at the University of Massachusetts-Dartmouth in fall 2011. They hung out in and outside school, with Kadyrbayev noting that he'd repeatedly visited Tsarnaev's home in Cambridge and met his family members.
FBI agents questioned Kadyrbayev and Tazhayakov for about 10 hours on April 19, then they were taken into custody the next day, said Robert Stahl, Kadyrbayev's lawyer. They remained detained for several days on alleged visa violations.
The FBI searches Dzhokhar Tsarnaev's dorm room in Pine Dale Hall, on the campus of the University of Massachusetts-Dartmouth. Among other things, they seize BB's, "a large pyrotechnic," as well as a black jacket and white hat like those worn on Marathon day by the man referred by authorities to as "Bomber Two."
Law enforcement agents recover Tsarnaev's backpack at a New Bedford landfill. Inside, they find an assortment of fireworks, a jar of Vaseline, a homework assignment sheet from the University of Massachusetts-Dartmouth and other things.
Federal authorities announce charges against three of Dzhokhar Tsarnaev's friends.
Azamat Tazhayakov and Dias Kadyrbayev are charged with having willfully "conspired with each other to commit an act against the United States ... by knowingly destroying, concealing and covering up objects belonging to Dzhokhar Tsarnaev."
Phillipos is charged with having "knowingly and willfully (made) materially false statements to federal investigators."