A Southern California man with ties to the white supremacy movement is the subject of a domestic terrorism investigation, federal authorities wrote in court documents.

The probe came to light in documents filed Friday in a separate case charging Brian Thibodeau with possession of an unregistered gun silencer. The 20-year-old has pleaded not guilty in federal court in San Diego in that case and is free on bond.

The FBI told special agents from the Department of Homeland Security who were investigating the silencer that Thibodeau was facing an domestic terrorism investigation involving so-called racially motivated violent extremists, the court documents say.

The FBI said their investigation involves white supremacy and domestic terrorism movements, the court documents say, as well as online planning for possible violent attacks in the U.S.

Though there’s a domestic terrorism investigation, no specific domestic terrorism law exists in the federal criminal code. That means the government must rely on other laws, such as hate crimes and weapons offenses, in politically motivated cases.

A spokesperson for the U.S. attorney’s office in San Diego declined to comment on Thibodeau’s case Monday. His attorney and an FBI spokesperson did not return requests for comment.

Agents executing a search warrant at Thibodeau’s home in Imperial, California, in March discovered the silencer, two firearms, a kit for making an AR-type rifle and metal pipes that authorities believe might be used to make pipe bombs. They also found six cellphones, three desktop computers, a laptop and an iPad.

In addition, officials uncovered two notebooks filled with racist and Nazi phrases and drawings, as well as various racist and antisemitic documents and propaganda items, a novelty grenade and a copy of “The Turner Diaries,” an antisemitic novel that is popular with extremists. The search warrant also turned up a T-shirt featuring a neo-Nazi symbol, a German helmet and a black Nazi-style jacket with skull and crossbones on the collar.

The Homeland Security investigation began on March 19, documents say, when U.S. Customs and Border Protection officials found the silencer at the Los Angeles International Mail Facility. It was addressed to Thibodeau.

An undercover agent delivered the silencer to Thibodeau on March 26. Federal authorities searched his home — about 15 miles (24 kilometers) north of the U.S.-Mexico border — and interviewed Thibodeau about his purchase.

He told the Homeland Security agents he had ordered the part online for $25 and it had been marketed as a fuel filter. He said he was going to use it to clean his rifle, the court documents say.

Thibodeau told the agents that the FBI had come to his home a few months earlier and tried to speak with him but he refused, according to the documents. He said he believed it was related to something he had posted online but he wasn’t sure. The agents later contacted the FBI, which told them of the domestic terrorism investigation.

When the agents told Thibodeau that he would face a state felony citation for the silencer, he suddenly raised his voice. He called the agents “federal pigs” and said it was a waste of time, the documents say. He later apologized.

Federal authorities on Friday applied for a warrant to search Thibodeau’s computers, iPad and cellphones.

If he is convicted of the federal silencer charge, Thibodeau would have to forfeit a Glock handgun, an AR-15 style rifle, nearly 500 rounds of various ammunition and several magazines.

He is scheduled to appear in court May 27 in an online hearing.