Mar 31, 2015 4:42 PM by CBS/AP
BOSTON -- Lawyers for the Boston Marathon bomber rested their case Tuesday after an FBI fingerprint examiner testified that Dzhokhar Tsarnaev's fingerprints weren't found on any of the marathon bomb components, but his older brother's were.
Elaina Graff said Tamerlan Tsarnaev's prints were found on two pieces of cardboard that came from one of the bombs used at the marathon.
She said prints belonging to both brothers were found on another bomb recovered from the scene of a gun battle with police days after the marathon attack. That bomb, which was not detonated, consisted of explosive powder and fuses packed inside a plastic container with a fuse protruding from the lid.
Tsarnaev's lawyers ended their case Tuesday after calling just four witnesses.
CBS Boston: Live updates from Boston Marathon bombing trial
Three people were killed and more than 260 were injured when two pressure cooker bombs exploded near the finish line.
The questioning was part of an effort by Dzhokhar Tsarnaev's lawyers to show that Tamerlan Tsarnaev was the driving force behind the 2013 terror attacks. Tamerlan Tsarnaev, 26, died after he was shot by police and run over by Dzhokhar, then 19, during a violent confrontation days after the bombings.
Under cross-examination from a prosecutor, Graff said it can be difficult to find fingerprints at bombing sites. "Due to the extreme temperature and force in an explosion, it is not unusual to not find fingerprints on items," Graff said.
Graff said only Tamerlan's prints were found on other items, including a jar of nails, tape, a caulking gun and other tools in the Tsarnaev family apartment in Cambridge.
During opening statements at the trial, Tsarnaev's lawyer, Judy Clarke, admitted he participated in the attacks but said he had fallen under the influence of his radicalized brother. The defense has made it clear that its goal is not to win an acquittal for Tsarnaev but to convince the jury that he is less culpable than his brother and does not deserve the death penalty.
If Tsarnaev, now 21, is convicted - a near foregone conclusion because of his lawyer's admission - the same jury will hear evidence in a second phase of the trial - the penalty phase - to decide whether he should be executed or spend the rest of his life in prison.
Also testifying for the defense Tuesday was a computer expert who said Tamerlan Tsarnaev did Internet searches on bombing components in the weeks before the 2013 terror attack.
Mark Spencer, the president of a digital forensics company, said search terms on Tamerlan Tsarnaev's laptop included "detonator, "transmitter and receiver" and "fireworks firing system." He said Dzhokhar Tsarnaev's laptop showed his computer activity focused heavily on Facebook and a Russian version of Facebook.
On Monday, prosecutors rested their case after jurors in his federal death penalty trial saw gruesome autopsy photos and heard a medical examiner describe the devastating injuries suffered by the three people who died in the terror attack.
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