An 18-year-old Alaska man has been arrested after taking to social media to declare “Let’s beat Hitler’s kill count” and referencing “shooting up” a synagogue—all while allegedly making homemade silencers and buying parts for a machine gun.

Prosecutors say the man, Michael Lee Graves, showed all the signs of a mass shooter preparing for a massacre, and a federal judge on Tuesday ordered that he remain in custody, according to the Anchorage Daily News.

In an affidavit unsealed Tuesday, the FBI described how it received a tip in late April that Graves may commit an act of violence related to the racist views he spouted on social media. Agent Joshua Rongitsch noted that Graves’ social media posts also make “many mentions of guns and acquiring guns and large capacity magazines.”

In addition to referencing Hitler, Graves allegedly posted an image of a person with a serious head injury alongside the caption “The perfect treatment for Muslims. Fixes them up good and new.”

While Graves is said to have later told investigators the social media posts were meant in jest, “he couldn’t articulate a good response as to why that would ever be funny to anyone,” Rongitsch testified in an Anchorage court on Tuesday.

The social media posts were what first put Graves on the FBI’s radar, but on the same day the tip about him was received, the agency said it also received notice from the Bureau of Alcohol, Tobacco, Firearms, and Explosives about a package intercepted by U.S. Customs and Border Protection addressed to Graves that contained an illegal device intended to convert a “semi-automatic Glock-type pistol into a machine gun.”

While searching his apartment, investigators reportedly discovered another 11 firearms and two homemade silencers, one of which featured an image of a swastika along with the number 1488, a code commonly used by neo-Nazis to reference Hitler and Nazi ideology.

“We have a lot of people in our community who he has avowed he wants to shoot up or kill,” Assistant U.S. Attorney Kimberly Sayers-Fay testified in court.

Graves’ attorney, Allen Dayan, told the judge the only thing his client was guilty of was being “a loudmouthed kid who likes guns and spouts off on social media.”

Graves will remain in custody until his trial on charges of possessing a machine gun and possessing an unregistered firearm. A trial date has not yet been announced.