The judge, George A. O’Toole Jr. of Federal District Court, limited the degree to which the defense could bring up the defendant’s older brother in this phase of the trial, which decides guilt or innocence. But Tamerlan seemed to hover over the proceedings, present even in death, as the families of victims who died and survivors who were maimed by the blasts crowded into the packed courtroom here as the much-anticipated trial finally got underway.

Perhaps anticipating the defense’s approach, the government’s lawyers, who spoke first, sought preemptively to smother any sympathy Ms. Clarke might stir up for her client.

Image Dzhokhar Tsarnaev Credit... Uncredited/Federal Bureau of Investigation, via Associated Press

William Weinreb, an assistant United States attorney, offered searing descriptions of how the bombs had ripped the flesh off spectators at the marathon and sent body parts flying in the air.

He described the precise way in which shrapnel had lacerated the three victims who died, as their families listened. Martin Richard, 8, “bled to death” on the sidewalk; Lingzi Lu, 23, had “the inside of her stomach pouring out”; and Krystle Campbell, 29, had “gaping holes” in her body. The government later called five eyewitnesses, three of whom suffered grievous injuries. All described the carnage after the bombing, their testimony accompanied by gruesome photos and video of victims lying in pools of blood.

“I could feel my body going tingly, and I was getting increasingly cold, and I knew I was dying,” said Sydney Corcoran, 19, whose femoral artery was severed by the blasts, causing her to lose most of her blood.