By Scott Malone and Elizabeth Barber

BOSTON (Reuters) - Lawyers for accused Boston Marathon bomber Dzhokhar Tsarnaev wrapped up their case on Tuesday after calling only four witnesses over two days.

By comparison, the prosecution presented 92 witnesses in the trial of Tsarnaev, 21, who is accused in the April 15, 2013, attack that killed three people and injured 264 people near the finish line of the famed race.

Court will reconvene on Monday, with lawyers for each side making their closing statements before the jury begins deliberations.

If Tsarnaev is found guilty, the jury will decide whether to sentence him to death or life in prison without possibility of parole after additional witness testimony.

In all, testimony in the guilt phase of Dzhokhar's trial took 16 days, less time than it took to pick the 12 jurors and six alternates.

On Tuesday, Tsarnaev's attorneys elicited testimony that the defendant's brother researched radio transmitters and guns in the weeks before attack. The jury also heard testimony that a laptop owned by the younger Tsarnaev, showed no such searches over that time period, with its user mostly surfing social-media sites including Facebook.

Digital forensics expert Mark Spencer testified about data found on three computers owned by the Tsarnaev brothers.

The computer belonging to Tamerlan, who died four days after the attack, 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.

Under cross-examination by federal prosecutors, Spencer acknowledged that Dzhokhar Tsarnaev's laptop showed Internet searchers for the phrase "the call of jihad" four days before the bombing.

Spencer also agreed that it was not possible to tell who had performed each Internet search and that Dzhokhar Tsarnaev at times had logged on to Tamerlan's computer.


Tsarnaev's lawyers opened the trial early this month by bluntly admitting that their client committed the crimes of which he 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 Tsarnaev 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.

The defense also questioned an FBI fingerprint examiner who processed evidence seized at Tamerlan Tsarnaev's Cambridge, Massachusetts, apartment. The examiner, Elena Graff, testified that she found Tamerlan's fingerprints, but not Dzhokhar's, on rolls of duct tape, a caulk gun and a glass jar filled with small nails.

Prior prosecution witnesses suggested those items were used in making the bombs.

Likewise, one of Tamerlan's fingerprints but none of Dzhokhar's were found on a piece of cardboard from one of the bombs that exploded on Boylston Street, Graff said.

Under cross-examination, Graff explained that it is not unusual to find few fingerprints on the remains of a bomb.

"Prints are usually left in the sweat or the oil that covers your fingers. They’re very fragile," Graff said. "Due to the extreme temperatures and force in an explosion, it is not unusual to not find fingerprints on items."

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)