Story highlights Edgar Welch allegedly traveled to a DC pizzeria to "investigate" a false story about a child sex slavery ring

Welch pleaded guilty Friday to federal and local gun charges

Washington (CNN) The North Carolina man who police say fired an assault rifle in a Washington, DC, pizzeria while investigating an online conspiracy theory known as "Pizzagate" pleaded guilty Friday to gun charges in federal court.

Edgar Maddison Welch, 28, was indicted by a federal grand jury in December after prosecutors said he marched into the Comet Ping Pong restaurant in northwest Washington and fired multiple shots from his AR-15 assault rifle, all in an apparent attempt to find and rescue child sex slaves that he believed were being held at the restaurant -- a belief allegedly based on his reading of a false story circulating online at the time.

After Welch found no evidence of child sex-trafficking at the restaurant, he did not harm anyone and surrendered, according to court documents.

Welch has pleaded guilty to a federal charge of interstate transportation of a firearm and a local DC charge of assault with a dangerous weapon. Prosecutors and the defense team have agreed that Welch faces a likely range of 18 to 24 months in prison for the federal charge and 18 to 60 months for the local offense under the applicable sentencing guidelines.

US District Court Judge Ketanji Brown Jackson has scheduled Welch's sentencing for June 22. He has been in custody since his arrest in December.