The round that struck a U.S. Secret Service officer during Saturday's shooting outside the ballroom where the White House Correspondents' Dinner was taking place was not friendly fire, multiple senior law enforcement officials told CBS News Wednesday.

Justice Department investigators, working with the Bureau of Alcohol, Tobacco, Firearms and Explosives, have at this point determined that the suspected gunman, 31-year-old Cole Allen, fired his shotgun, the officials said.

A source familiar with the investigation previously told CBS News that a total of six shots were fired when the gunman charged a security checkpoint at the Washington Hilton — one by the suspect, and the other five rounds by the Secret Service officer who was struck in his bulletproof vest.

Two sources familiar with the investigation said the shot that struck the agent likely hit the cellphone that was tucked inside a pocket of his bulletproof vest. The officer was not seriously hurt.

Allen, of Torrance, California, fell to the floor during the incident and was arrested at the scene, officials said. He was not wounded during the incident.  

He has since been charged with attempting to assassinate the president of the United States, discharging a firearm during a crime of violence, violent crime and transporting a firearm in interstate commerce with intent to commit a felony.

He made his first federal court appearance Monday, but did not enter a plea.