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The Associated Press
In this courtroom sketch, Judith Russell, mother-in-law of Tamerlan Tsarnaev, is depicted testifying during the penalty phase in the trial of Dzhokhar Tsarnaev. Monday, April 27, 2015, in federal court in Boston. Dzhokhar Tsarnaev was convicted of the Boston Marathon bombings that killed three and injured 260 people in April 2013. His brother Tamerlan Tsarnaev died during a firefight with police four days after the bombings.

BOSTON — As a young child, Boston Marathon bomber Dzhokhar Tsarnaev was quiet, hardworking and "always wanted to do the right thing," his third-grade teacher testified Wednesday to jurors who will decide whether he spends the rest of his life in prison or is sentenced to death.

Catheryn Charner-Laird testified on the third day of the defense case in the penalty phase of Tsarnaev's trial as his lawyers shifted the focus away from his older brother, Tamerlan. The defense has portrayed Tamerlan Tsarnaev, who died days after the bombing, as the mastermind of the attack.

Three people were killed and more than 260 were wounded when twin pressure-cooker bombs exploded near the finish line of the marathon on April 15, 2013.

Tsarnaev, 21, was convicted of all 30 charges against him, including 17 that carry the possibility of the death penalty. The same jury must now decide his punishment.

Tsarnaev's defense team has focused heavily on Tamerlan, arguing he was a domineering influence on Dzhokhar and led him down the path to terrorism.

But on Wednesday, Tsarnaev's lawyers began calling witnesses to testify about what he was like as a child, years before he became the Boston Marathon bomber.

"He was just learning English at that time," Charner-Laird said, referring to Tsarnaev's recent move to the U.S. from Russia with his family.

Tsarnaev was 9 in the fall of 2002 when he was one of her students in a combination class for third- and fourth-graders at the Cambridgeport School, she said.

"He was incredibly hardworking," she said. "He cared a lot about his studies; he tried very hard."

Many times, he didn't know what to do because of the language barrier, she said. But he "always wanted to do the right thing," she said.

"He never seemed to stop or give up," she said.

Prosecutor Aloke Chakravarty cross-examined her briefly, asking if she knew Dzhokhar to be disciplined and smart. She said he was. The question appeared designed to rebut the defense claim that Dzhokhar was under Tamerlan's influence when he participated in the bombings.

Prosecutors have said the brothers were partners in the bombings, which were designed to retaliate against the U.S. for its actions in Muslim lands.

The defense also showed the jury two photographs of a cherubic-looking Tsarnaev from about the same time. In one of the photos, he is sitting on a bench next to Tamerlan, who is about 16. Tamerlan has his arm around him, while Dzhokhar rests his arm on his older brother's leg.

In another photo, Dzhokhar is shown smiling with his mother, two sisters and the landlady who owned the Cambridge apartment building where they lived.