As jurors looked at a photograph of Boston Marathon bomber Dzhokhar Tsarnaev giving the finger to a security camera in his jail cell, a federal prosecutor described it as a defiant act by an unrepentant man who didn't care that he had killed four people, including an 8-year-old boy and a police officer.

Assistant U.S. Attorney Nadine Pellegrini began laying out the government's case for executing Tsarnaev, and showed the jury large, vibrant pictures of the people killed in the bombing and its aftermath. Then she revealed the photo of Tsarnaev, taken three months later in his holding cell at the federal courthouse.

"This is Dzhokhar Tsarnaev — unconcerned, unrepentant and unchanged," Pellegrini on Tuesday told the jurors who will decide whether the 21-year-old former college student should be executed.

The penalty phase in the Boston Marathon bomber's trial opened in dramatic fashion, with prosecutors portraying Tsarnaev as a coldblooded killer and "America's worst nightmare."

The government then began trying to drive home the horror of the attack by calling to the stand witnesses who lost legs or loved ones in the April 15, 2013, bombing.

"I remember hearing just bloodcurdling screams. I just remember looking around, just seeing blood everywhere, sort of like debris falling from the sky," said Celeste Corcoran, who made her way to the stand on two artificial limbs.

Several jurors shed tears as the father of Krystle Campbell, a 29-year-old restaurant manager killed in the bombing, described how he called his daughter "princess."

"Krystle was the light of my life," William Campbell Jr. said, "every father's dream."

He wiped away tears with a handkerchief, his voice growing hoarse as he described how she "wasn't really a girly-girl" and preferred baseball over other activities.

Campbell described a heartbreaking mix-up that led his family to believe that Krystle had survived the bombing and was undergoing surgery. One of the doctors asked Campbell to take a walk with him to go see Krystle in her room.

"It wasn't Krystle. I passed out on the floor," Campbell said. "I couldn't remember anything after that until I woke up about five minutes later and I realized that Krystle was gone and they made a mistake."

Earlier Tuesday, prosecutors showed the jury a photo of a wounded Krystle writhing in agony on the ground, her mouth agape.

Slouching in his seat at the defense table as usual, Tsarnaev stared straight ahead and showed no reaction during the proceedings.

Three people were killed and more than 260 wounded in the bombing, carried out by Tsarnaev and his now-dead older brother, Tamerlan, to punish the U.S. for its wars in Muslim countries. A Massachusetts Institute of Technology police officer was shot to death days later as the brothers tried to get away.

Tsarnaev was convicted earlier this month of all 30 charges against him during the trial's guilt-or-innocence phase. His lawyers did not give an opening statement Tuesday but will do so once the prosecution has made its case.

The defense contends Tamerlan, 26, masterminded the bombing, and Dzhokhar, then 19, fell under his influence.

The 12-member jury must be unanimous for Tsarnaev to receive a death sentence; otherwise, he will automatically get life behind bars.

Prosecutors have argued that Tsarnaev was a full partner with his brother and deserves the ultimate punishment.