BOSTON — The mother of a Massachusetts Institute of Technology police officer killed by Boston Marathon bomber Dzhokhar Tsarnaev was so distraught by his death that she could not get out of bed for months, the officer’s stepfather testified Wednesday.

Officer Sean Collier was gunned down three days after the bombing as Tsarnaev and his brother, Tamerlan, were trying to flee.

Sean Collier had worked for the MIT police for a little over a year when he was shot to death pm April 18, 2013, on the school's campus in Cambridge, Mass., by Tamerlan and Dzhokhar Tsarnaev. AP

In this July 10, 2013, surveillance photo released by the U.S. Attorney's office, Dzhokhar Tsarnaev extends his middle finger in an obscene gesture to a security camera in his jail cell in Devens, Mass. The photo was presented during the penalty phase of Tsarnaev's trial on Tuesday as a federal prosecutor laid out the government's case for executing him. The Associated Press/U.S. Attorney's Office

Tsarnaev, 21, was convicted earlier this month in the bombing and Collier’s killing. The jury is now hearing testimony on whether Tsarnaev should be put to death.

Collier’s stepfather, Joseph Reynolds, described a close-knit, blended family that formed when he and Collier’s mother married in 1993. Reynolds said he raised Collier from about the age of 6.

“It was sort of a ‘Brady Bunch’ situation,” he said.

Reynolds, as well as Collier’s younger brother, Andrew, described Sean as a kid who always had a strong sense of right and wrong, down to putting bugs outside rather than killing them.

“He was a cop at an early age,” Reynolds said, testifying on the second day of the penalty phase of the trial.

Three people were killed and more than 260 wounded in the 2013 bombing.

Tsarnaev’s lawyers say Tamerlan masterminded the bombing and recruited his impressionable younger brother, then 19. They argue that Dzhokhar does not deserve the death penalty.

Prosecutors showed the jury a series of photographs of Collier as a child and a picture of him at his graduation from the police academy, his mother pinning his badge on him.

“That was probably the happiest day of his life,” Reynolds said.

Prosecutors say the Tsarnaev brothers shot Collier in his police cruiser during a failed attempt to steal his gun. Hours earlier, the FBI had released photos of the brothers as suspects in the marathon bombing.

Reynolds said Sean’s mother has been diagnosed with post-traumatic stress disorder and has not been able to return to work.

“It’s been a terrible two years,” he said.

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