A Savage man who was allegedly preparing for a violent confrontation with police pleaded guilty May 16 in U.S. District Court to the unlawful possession of a machine gun.

River William Smith, 21, was arrested by FBI agents in December in the Savage Cub Foods parking lot and was later indicted by a federal grand jury on charges related to the illegal possession of firearms.

According to the U.S. Attorney’s office, an investigation revealed that Smith said he was “pro mass shooting” and sympathized with mass shooters and that evidence showed he had been planning a violent exchange with police. When he was arrested, authorities also found Smith had access to an AR-style rifle, handguns, body armor and a Kevlar helmet.

Tips from citizens concerned about Smith’s behavior led to the probe, according to the U.S. Attorney’s office. The FBI used a confidential source to communicate with Smith and Smith was arrested after trying to buy weapons parts from the source. The Savage Police Department worked with the FBI on the investigation.

A sentencing hearing will be scheduled at a later date.