BOSTON -- First, the jury was shown large, vibrant pictures of the four people killed in the Boston Marathon bombing and its aftermath. Then prosecutors pulled out the photo they saved for last: Dzhokhar Tsarnaev giving the finger to the security camera in his jail cell.

"This is Dzhokhar Tsarnaev -- unconcerned, unrepentant and unchanged," federal prosecutor Nadine Pellegrini told the jury that will decide whether the 21-year-old former college student should be executed.

The penalty phase in Tsarnaev's trial opened in dramatic fashion yesterday, with prosecutors portraying him as a cold-blooded killer and "America's worst nightmare." The government tried 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.

Slouching at the defense table, Tsarnaev stared ahead and showed no reaction during proceedings. He did not appear to look at any of the witnesses.

Three people were killed and more than 260 wounded in the bombing, carried out by Tsarnaev and his older brother, Tamerlan. An MIT police officer was shot to death days later as the brothers tried to flee.

Tsarnaev's lawyers did not give an opening statement yesterday but will do so once the prosecution has made its case.

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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.