Defense to Intensify in Trial of Accused Boston Marathon Bomber

Defense of accused Boston Marathon bomber Dzhokhar Tsarnaev is expected to intensify on Tuesday with his lawyers' first full day of calling witnesses.

Tsarnaev, 21, could be sentenced to the death penalty or life in prison if found guilty of killing three and injuring 264 people in the April 15, 2013 attack.

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 Tsarnaev's brother, 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. US 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. Both were killed by the bomb Tsarnaev is accused of leaving near the finishing line of the race.

The other fatality on the day of the marathon was restaurant manager Krystle Campbell, 29. Massachusetts Institute of Technology police officer Sean Collier, 26, was shot dead on April 18 as the brothers prepared to flee the city.

Defense attorneys called two witnesses on Monday, starting with a photographer for the Federal Bureau of Investigation, who testified that a book on home wiring was found in Tamerlan Tsarnaev's apartment.

Computer forensics expert Gerald Grant said records showed that Tsarnaev's mobile phone was near his college south of Boston at the times prosecutors contend the pressure cookers and BB pellets used in homemade bombs were purchased more than an hour's drive away, suggesting the defendant could not have bought them.

Grant also testified about Tsarnaev's Twitter activity, which has been called into question by prosecutors who introduced into evidence a tweet Tsarnaev sent on the day of the Boston Marathon a year before the bombing. It read, "They will spend their money and they will regret it and then they will be defeated."

Grant testified that Tsarnaev's next tweet that day, about two hours later, was banal. It read, "Hhmmm get breakfast or go back to sleep, this is always a tough one."