Accused Boston Marathon bomber Dzhokhar Tsarnaev's older brother, Tamerlan, researched radio transmitters, fireworks and gun stores in the weeks leading up to the 2013 attack that killed three people, a computer expert testified on Tuesday.
A laptop owned by the younger Tsarnaev, who could be sentenced to death if he is convicted of carrying out the attack, showed no such searches over that time period, with its user mostly surfing social-media sites including Facebook.
Lawyers for Dzhokhar Tsarnaev, 21, on Tuesday called their third witness on behalf of the defendant, digital forensics expert Mark Spencer, who testified about data found on three computers owned by the Tsarnaev brothers.
Prosecutors contend that the brothers used remote controls from toy cars to detonate the homemade pressure-cooker bombs stuffed with gunpowder extracted from fireworks that ripped through the crowd at the race's finish line, and three days later used a Ruger P95 handgun to shoot dead Massachusetts Institute of Technology police officer Sean Collier.
They have contended that 26-year-old Tamerlan was the driving force behind the bombing and that Dzhokhar played a secondary role in it and in the fatal shooting of a police officer three days later.
Tamerlan died early on April 19, 2013, after Dzhokhar inadvertently ran him over with a hijacked SUV at the end of a gunfight with police.
The other fatality on the day of the marathon was restaurant manager Krystle Campbell, 29.
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