A still image captured from surveillance video at the Boston Marathon shows the scene moments before a second bomb exploded as a man, marked with a circle by prosecutors (top R) and identified by them as defendant Dzhokhar Tsarnaev, moves rapidly away from the spot near the finish line of the race on April 15, 2013, in this handout video provided by the US Attorney's Office in Boston, Massachusetts on March 9, 2015. [Photo/Agencies]

BOSTON - Moments after being blasted into the air "like a rocket" in the Boston Marathon bombing, nurse Jessica Kensky saw that her husband's leg had been blown off and leaped into action, struggling to put a tourniquet on him, she testified on Monday.

Running on adrenaline, she did not realize the blast from one of the two homemade pressure-cooker bombs had set her back on fire, Kensky told jurors in the third day of accused bomber Dzhokhar Tsarnaev's federal trial in Boston.

"There was smoke, there was blood. I was most focused on my husband, he was right next to me still, and his foot and his leg were kind of detached," said Kensky. "A man came over as I was trying to fumble to put a tourniquet on Patrick and said, 'Ma'am you're on fire, you're on fire.'"

Tsarnaev, 21, is accused of killing three people and injuring 264, including Kensky, at the famed race's crowded finish line on April 15, 2013.

Kensky lost her left leg on the day of the bombing. Her right leg was amputated in January of this year because it had failed to heal from injuries sustained in the blast.

She and her husband, Patrick Downes, were taken to separate hospitals and did not see each other for two weeks until a volunteer medical crew brought her to visit him.

Tsarnaev's attorneys opened their case last week by bluntly declaring that the defendant and his older brother were responsible for the attack as well as the fatal shooting of a police officer three days later, in an effort to focus attention on the brother's role in the plot.