Some of the Marines who were shot to death last week in Chattanooga, Tenn., effectively sacrificed themselves to save the lives of others, diverting the gunman away from a larger group of potential victims, according to a law enforcement official briefed on the investigation into the killings.

“This could have been a lot worse,” said the official, who did not want to be identified because he was not authorized to discuss the investigation. “It could have been a horrible, horrible massacre — so much worse.”

Law enforcement officials are expected to give a more detailed account of the deadly attack on two military sites last Thursday at a news conference scheduled for later on Wednesday.