Mystery deepens over assault on Sen. Rand Paul FBI and local authorities are investigating the potential federal crime.

 -- Kentucky Sen. Rand Paul is recovering at his Bowling Green home after being physically assaulted Friday afternoon by a neighbor. The assailant – 59-year-old Rene Boucher – was arrested by Kentucky State Police and charged with fourth-degree assault, authorities said.

U.S. Capitol Police and the FBI are investigating the incident because assaulting a member of Congress is a federal crime.

"Senator Paul was blindsided and the victim of an assault,” his spokeswoman said Friday in a statement, adding that the senator is “fine.”

Doug Stafford, a senior adviser to Paul, said in a statement that the senator sustained five rib fractures, caused by high-velocity severe force and lung contusions. Stafford added that it is unclear when Paul would return to work because the pain associated with the injuries is considerable and could last weeks or months.

“It is a pending, serious criminal matter involving state and federal authorities. We won’t have any further comments at this time,” Stafford told ABC News.

According to the arrest warrant, Boucher walked onto Paul’s property and tackled him from behind, forcing him to the ground.

Matthew Baker, Boucher's attorney, said in a statement to ABC News Monday, "Senator Paul and Dr. Boucher have been next door neighbors for 17 years. They are also prominent members of the local medical community and worked together when they were both practicing physicians."

It continued, "The unfortunate occurrence of Nov. 3 has absolutely nothing to do with either's politics or political agendas. It was a very regrettable dispute between two neighbors over a matter that most people would regard as trivial. We sincerely hope that Senator Paul is doing well and that these two gentlemen can get back to being neighbors as quickly as possible."

The FBI said in a statement that it is working with state and local partners to determine if there was a violation of federal law. U.S. Capitol Police said it would not comment on the ongoing investigation.