As emergency responders and marathon volunteers and runners attended to the wounded, the Tsarnaevs were captured on surveillance videos calmly hustling away from the carnage.
On April 18, the FBI released photographs of the two men, then unidentified, in baseball caps along with a warning that they were "armed and dangerous."
Once their photos were disseminated, the Tsarnaev brothers "armed themselves with five IEDs, a Ruger P95 semiautomatic handgun, ammunition for the Ruger, a machete, and a hunting knife and drove to the Massachusetts Institute of Technology," the indictment says.
That's where MIT Police Officer Sean Collier was just 35 minutes away from finishing his shift. The indictment says Tamerlan Tsarnaev shot him in the head at "close range" with the Ruger 9mm handgun as his younger brother tried to steal Collier's service revolver from his gun belt.
After the officer's murder, the Tsarnaev brothers are accused of carjacking and kidnapping another man. The carjacking victim told police that Tamerlan Tsarnaev pointed a gun at him and said: "Did you hear about the Boston explosion? I did that," according to an affidavit filed by FBI agent Daniel R. Genck.
Tsarnaev then climbed into the Mercedes SUV and removed the gun's magazine, showed the driver the round in the chamber, and then reloaded the weapon and added, "I am serious."
The victim was driven around to collect money from various ATMs but claimed he escaped at a gas station and called 911. Police used that man's cell phone – left behind in the stolen car – to track the brothers to Watertown where a chase ensued.
During the chase, the brothers hurled two homemade bombs onto Dexter Street at police and then "a gun fight ensued between the car's occupants and law enforcement officers,'' according to Genck's affidavit.
After Tsarnaev fled the Watertown gun battle just before 1 a.m. ET on April 19, a manhunt was launched in the blue-collar suburb. He was found later that day wounded and bleeding in a boat dry-docked in a Watertown resident's backyard.
"He had visible injuries, including apparent gunshot wounds to the head, neck, legs and hand,'' the affidavit states. The one-time nursing student was carrying his University of Massachusetts/Dartmouth ID card when he was pulled from the boat.
As he hid, Tsarnaev "wrote a message on an inside wall and beams of the boat that said (among other things): 'We Muslims are one body, you hurt one, you hurt us all,''' the indictment states.
Massachusetts law enforcement sources told ABC News that one of the "other things" mentioned in the indictment were the words "F--- America." Investigators recovered additional explosives in two cars -- the green Honda and the Mercedes SUV -- left in Watertown by the Tsarnaev brothers, the affidavit states.