By Elizabeth Barber

BOSTON (Reuters) - When ballroom dancer Adrianne Haslet-Davis heard a blast as she stood at the finish line of the Boston Marathon two years ago, she knew it was no accident and suspected another explosion would follow.

But she had no idea the second bomb was beside her until it went off with a deafening blast and a blinding flash of light.

"I couldn't hear myself scream," Haslet-Davis on Wednesday told the jurors who will determine if convicted bomber Dzhokhar Tsarnaev is sentenced to death or life in prison. "I thought that because I couldn't hear myself scream, I was dead."

Federal prosecutors called a series of witnesses who described what they lost when the 21-year-old ethnic Chechen and his brother set off a pair of homemade pressure-cooker bombs at the race's crowded finish line on April 15, 2013. The bombing killed three people and injured 264, including 17 who lost legs.

Haslet-Davis, 34, was among them. She broke down in tears while describing a day that she began in platform heels, trying to "look cute" for her husband, and ended on the ground, her left leg soaked with blood.

Her husband, Adam, had applied the first tourniquet to her wounded leg before the two realized that he was also seriously wounded. When she was separated from him and brought to a hospital where her left leg was amputated, she believed he was dead and that she would also die, she testified.

She recalled calling her parents.

"I told them Adam is dead, and this might be it for me," Haslet-Davis said.

Adam survived the attack, but has since "bravely checked himself into" a mental hospital, Haslet-Davis testified.

In addition to the three people killed by the bombs, 8-year-old Martin Richard, 23-year-old Chinese graduate student Lingzi Lu and 29-year-old restaurant manager Krystle Campbell, the Tsarnaev brothers shot dead Massachusetts Institute of Technology police officer Sean Collier three days later as they prepared to flee the city.

Tamerlan Tsarnaev, 26, died hours later after Dzhokhar ran him over with a car at the end of a gunfight with police.

Lu's aunt, Jinyan Zhao, testified that her niece was a "beautiful nerd" who had long wanted to study in the United States.

"America was her first choice," said Zhao, who added that Lu's parents had traveled to Boston from Shenyang, China, for their daughter's funeral but were too devastated by the loss of their only child to make the trip a second time.


Collier had dreamed of wearing a badge and driving a police cruiser since childhood, his brother and father testified.

"He was a typical little boy, wanted to be a police officer," said Collier's 27-year-old brother, Andrew. "But he never grew out of it."

Joseph Rogers, Collier's stepfather, said the day he graduated from the police academy was "probably the happiest day of his life."

Rogers testified that his wife has stopped working and was diagnosed with post traumatic stress disorder after her son's slaying and that Collier's siblings now suffer from depression.

Defense attorneys have opted to delay their opening statements until next week when they begin calling their own slate of witnesses aimed at persuading the jury to spare their client's life.

Their argument is expected to focus on Tamerlan, who they have painted as the mastermind, with Dzhokhar following out of a sense of familial fealty rather than personal conviction.

Prosecutors have noted that Dzhokhar Tsarnaev left behind a note suggesting the attack was an act of retribution for U.S. military campaigns in Muslim-dominated countries.

In anticipation of that argument, lawyers on Wednesday sparred over the significance of surveillance footage in which Tsarnaev raises his middle finger at a camera in a courthouse holding cell while awaiting arraignment in July 2013.

Prosecutors used a still image of the vulgar gesture on Tuesday to argue that the defendant was evidently "unconcerned, unrepentant and unchanged," even after his brother was gone.

The jury on Wednesday viewed a video of the episode in which Tsarnaev is seen arranging his hair in the camera’s reflective mirror before rapidly flicking two fingers and then one.

In defense lawyers' first cross-examination of a witness during the sentencing phase of the trial, Miriam Conrad asked U.S. Marshal Gary Oliveira if he knew how old Tsarnaev was at the time of the video.

When Oliveira said he did not, Conrad replied, "You didn't know that he was 19 years old?"

(Editing by Scott Malone and James Dalgleish)