BOSTON — It’s “highly likely” that a police officer who was critically injured in a dramatic gunbattle with the Boston Marathon bombers was struck by friendly fire, a district attorney said Tuesday.

Middlesex District Attorney Marian Ryan, whose office has been investigating the gunfight between officers and Tamerlan and Dzhokhar Tsarnaev, said whatever wounded Massachusetts Bay Transportation Authority Officer Richard Donohue is still embedded in his leg, leaving investigators unable to confirm which gun fired the bullet.

“We don’t know what struck Officer Donohue because it has not been removed,” she said at a news conference. “It is likely that it came from the gun of one of the officers attempting to stop Dzhokhar Tsarnaev, yes.”

But Ryan said the shooting was justified because it was in “self-defense” of officers and people living in the dense residential neighborhoods where the gunfight took place on April 19, 2013.

The bullet that struck Donohue severed a major artery, and he lost most of his blood and suffered nerve damage. He returned to work last month.

Donohue said in a statement released by MBTA police that Ryan’s report “does not take away from the heroic actions of those officers who ultimately prevented further acts of terror.” He said, “I harbor no regrets and am forever grateful to the police officers, firefighters, and medical professionals who saved my life.”

The gunfight took place four days after the marathon.