Ronald Philip Lee Jr. and Patrick H. Sonsona, who were arrested at their homes by the FBI, allegedly tried to illegally modify assault rifles, the FBI said.

According to an FBI news release, on April 21, Lee knowingly accepted custody of a package containing an "auto-connector" from a U.S. postal inspector. This device convert certain semi-automatic rifles to fire fully-automatic mode, the FBI said.

Lee and Sonsona were charged with violating a federal law that makes it unlawful to possess a machine gun, which includes the possession of "any part designed and intended ... for use in converting a weapon into a machine gun," the FBI news release said.

A spokeswoman with the state Department of Public Safety confirmed the arrests but did not issue a statement.

She forwarded all calls to the FBI.