BOSTON (AP) — A ballistics expert has testified that 56 bullets were fired at police in Watertown during a violent confrontation with the Boston Marathon bombers days after the 2013 attack.

State police Lt. David Cahill testified Tuesday during the federal death penalty trial of Dzhokhar Tsarnaev.

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Cahill said 56 shell casings from a 9mm, semi-automatic Ruger handgun were found at the scene of a battle Tsarnaev and his brother, Tamerlan, had with police on April 19, 2013. Cahill said the same gun was used to kill MIT police Officer Sean Collier hours earlier.

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Dzhokhar Tsarnaev’s lawyers say it was Tamerlan Tsarnaev who shot Collier and used the gun during the battle with police. Dzhokhar is charged in the bombings, in Collier’s shooting and the Watertown confrontation. Tamerlan died following the shootout.

Three people were killed in the marathon bombings.

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