Edgar Maddison Welch, who fired shots in a Washington, D.C., pizza restaurant in December, was sentenced to four years in prison for federal and local crimes.
Welch, 29, was arrested for firing an AR-15 inside the D.C. restaurant Comet Ping Pong, as he investigated an unfounded conspiracy theory dubbed "Pizzagate" that the restaurant was involved in a child sex-trafficking ring connected to Hillary Clinton.
Welch "carried a loaded AR-15 assault rifle and a revolver into a Northwest Washington pizza restaurant, scattering employees and customers, and fired his assault rifle into a door," the U.S. Attorney's Office for D.C. said in a statement announcing the sentence.
Welch received four years for a federal charge of interstate transportation of a firearm and ammunition and two years for a District of Columbia charge of assault with a dangerous weapon, to be served concurrently. He was also sentenced to 36 months of supervised release. He pleaded guilty to the charges in March.
Judge Ketanji Brown Jackson called "the extent of the recklessness" of the defendant's actions "breathtaking" as she sentenced Welch on Thursday.
Jackson said she had no reason to doubt that Welch "thought" he was "being helpful," but said she could not "overstate" the concern that "other people will see what you have done and be inspired by it."
“No matter how well-intentioned, people are not allowed to take matters into their own hands,” she said.
Upon his release from prison, Welch will receive a mental health assessment. He was ordered to stay away from Comet Ping Pong.
Welch's defense attorney, Dani Jahn, gave a passionate plea for a lighter sentence for her client, saying that 18 months would be a "strong sentence."
“He is extremely remorseful and it is sincere and it is significant,” she added.
Welch spoke briefly in court, apologizing to everyone involved, including the restaurant staff.
Comet Ping Pong owner James Alefantis said in court that his staff was "brave" and that Welch caused him physical, emotional and financial harm.
“So many of us have suffered from the defendant’s actions,” he said.
ABC News' Julia Jacobo contributed to this report.