Rene Boucher, center, appears in court for an arraignment hearing with his attorney Matt Baker, left, on Thursday, Nov. 9, 2017, at the Warren County Justice Center in Bowling Green, Ky. Boucher pleaded not guilty to charges that he assaulted his neighbor, U.S. Sen. Rand Paul. Boucher and Paul have been neighbors for 17 years in a gated community. (Austin Anthony/Daily News via AP)

BOWLING GREEN, Ky. - U.S. Sen. Rand Paul's longtime neighbour 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.

Boucher's lawyer, Matt Baker, said the possibility of a federal charge "is a concern."

"I hope that that doesn't happen," Baker said. "And the research that I have done thus far would indicate that there's no applicability of the federal statute that appears to address the issue."

He said that he's still investigating the cause of the attack.

"It has nothing to do with politics," Baker said. "It's a property dispute between two neighbours."

A staffer with Paul's office disputed media reports that the two men had a "longstanding dispute."

"The Pauls have had no conversations with him in many years," senior advisor Doug Stafford said. "The first 'conversation' with the attacker came after Sen. Paul's ribs were broken. This was not a fight, it was a blind side, violent attack by a disturbed person."

A friend of Paul's told WKU Public Radio that she saw the senator after the attack and recounted his reaction.

"He said that when he got up, Rene said something like 'I've been trying to sell my house for 10 years and your trees are in the way,'" Alicia Stivers told the station.

Warren County District Judge Brent Potter set a pre-trial court date for Nov. 30. Boucher has been ordered to stay away from Paul and the senator's family.

Baker said it appeared that Paul had retained a personal injury attorney to represent him in the matter, which could mean he is considering a lawsuit. The lawyer identified by Baker did not return calls seeking comment, and Paul's office declined comment.

Boucher, 59, was released from jail on Saturday, a day after the attack, on a $7,500 bond.

Paul and Boucher have been neighbours for 17 years in a gated community.

Rob Porter, a close friend of the lawmaker, said this week that Paul doesn't know why he was attacked. Paul was mowing his yard and had stopped to remove a limb when Boucher tackled him from behind, Porter said. Paul was wearing ear protection and didn't hear Boucher coming, the friend said.

Other residents of the neighbourhood have issued statements praising Paul as a neighbour but do not appear to have witnessed the incident.

News from © The Associated Press, 2017