Jeanine Pirro, the top federal prosecutor for Washington, released the video on social media on Thursday.
She said it shows Cole Allen Tomas shooting a US Secret Service officer as he runs through security toward the gala packed with journalists, administration officials and others.
Pirro said there is no evidence that the agent was hit by friendly fire.
Allen agreed earlier on Thursday to remain jailed while he awaits trial.
Allen did not enter a plea during his brief appearance in federal court days after authorities say he ran through a magnetometer at the Washington Hilton while holding a long gun and disrupted one of the highest-profile annual events in the nation's capital.
Allen was injured during Saturday night's attack but was not shot. A Secret Service officer was shot but was wearing a bullet-resistant vest and survived, officials say.
Prosecutors have said they believe Allen fired his shotgun at least once and that a Secret Service agent fired five shots. They had not previously confirmed that it was Allen's bullet that struck the agent's vest.
In court papers pressing for Allen's continued detention, prosecutors wrote Allen took a picture of himself in his hotel room just minutes before the incident, and that he was outfitted with an ammunition bag, a shoulder gun holster and a sheathed knife.
In a message that authorities say sheds light on his motive, Allen referred to himself as a "Friendly Federal Assassin" and alluded obliquely to grievances over a range of Trump administration actions.
Allen's lawyers agreed during the brief hearing before US Magistrate Moxila Upadhyaya to keep their client behind bars for now after initially arguing in court papers that Allen should be released.
In a court filing, the defence wrote the US government's case is "based upon inferences drawn about Mr Allen's intent that raise more questions than answers" and noted that Allen's writings never mentioned Trump by name.
The defence left the door open to pressing in the future for Allen's release before trial.
Allen's lawyers alleged some of acting Attorney General Todd Blanche's statements "indicate that the recovered ballistics evidence is inconsistent with aspects of the government's theory, evidence collected by the government and/or statements made by witnesses."
The Justice Department, in response, said the evidence shows Allen fired his shotgun at least once in the Secret Service agent's direction.
Investigators recovered at least one fragment at the crime scene that is consistent with a buckshot pellet, prosecutors wrote.
"The government is aware of no physical evidence, digital video evidence, or witness statements that are inconsistent with the theory that your client fired his shotgun in the direction" of the officer or that the officer "was indeed shot once in the chest while wearing a ballistic vest," prosecutors wrote.
Allen was charged with that crime, as well as two additional firearms counts, including discharging a weapon during a crime of violence. He faces up to life in prison if convicted of the assassination count alone.
Allen, 31, is from Torrance, California. He is a highly educated tutor and amateur video game developer.
