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.