BOWLING GREEN, Ky.  — U.S. Sen. Rand Paul’s longtime neighbor pleaded not guilty Thursday to charges that he assaulted the Kentucky Republican while the senator was mowing his lawn.

Rene Boucher only spoke to reply “yes sir” to a judge during a brief arraignment. He showed no emotion and kept his head down. Boucher faces up to a year in jail if convicted of fourth-degree assault.

Paul, who was not at the hearing, wrote on Twitter on Wednesday that he suffered six broken ribs and has excess fluid around his lungs. The injuries have sidelined Paul from the Senate as he recovers at his home.

A prosecutor said after court that criminal charges may change because of the severity of the lawmaker’s injuries.

Warren County prosecutor Amy Milliken didn’t provide more details but said her office was working with Kentucky State Police and the FBI. She said the investigation “has not concluded.”

Milliken declined to discuss what could have caused the attack.