By Scott Malone and Elizabeth Barber
BOSTON (Reuters) - 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.
The computer belonging to Tamerlan, who died four days after the April 15, 2013, attack, which also injured 264 people, showed searches for information on radio transmitters and receivers and on Ruger P95 handguns.
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.
Tsarnaev's lawyers opened the trial early this month by bluntly admitting that their client committed the crimes of which stands accused, but left his formal "not guilty" plea in place, meaning a jury must first convict him before taking up the question of whether to sentence him to death.
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.
Their argument is intended to reduce Dzhokhar Tsarnaev's relative culpability in the jury's eyes and persuade them to sentence him to life in prison. U.S. District Judge George O'Toole has ruled that argument cannot be made in full until after the jury decides whether he is guilty.
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.
On Monday, prosecutors wrapped up their case against Tsarnaev with testimony from the medical examiners who autopsied 23-year-old Chinese exchange student Lingzi Lu and 8-year-old Martin Richard.
The other fatality on the day of the marathon was restaurant manager Krystle Campbell, 29.
(Reporting by Scott Malone; Editing by Nick Zieminski)