Federal prosecutors charge Ryan Routh with attempted assassination of Donald Trump

Federal prosecutors have officially charged Ryan Routh with attempting to assassinate former President Donald Trump.

The indictment adds three charges -- including attempted assassination of a major presidential candidate, possessing a firearm in furtherance of a crime of violence and assaulting a federal officer -- on top of the two federal firearms charges Routh was already facing.

The move was expected and previewed both by prosecutors in a court hearing on Monday and by Attorney General Merrick Garland in a news conference Tuesday afternoon.

"Violence targeting public officials endangers everything our country stands for, and the Department of Justice will use every available tool to hold Ryan Routh accountable for the attempted assassination of former President Trump charged in the indictment," Garland said in a statement. "The Justice Department will not tolerate violence that strikes at the heart of our democracy, and we will find and hold accountable those who perpetrate it. This must stop."

He is expected to be arraigned on the charges in a court hearing next week.

Judge Aileen Cannon, the Trump-appointed judge who threw out Trump’s classified documents case, has been randomly assigned to oversee the Ryan Routh case.

Routh, 58, had previously been charged with possession of a firearm as a convicted felon and possession of a firearm with an obliterated serial number for the incident that took place at Trump International Golf Club on Sept. 15.

On Monday, prosecutors outlined new details about their investigation and said there is "probable cause to support additional charges which can and should be considered by the court."

Routh possessed a list that included dates from August to October of venues where Trump had appeared or was expected to be -- and is suspected to have traveled near the golf course and Trump's Mar-a-Lago resort multiple times in the month leading up to his arrest, prosecutors said in a detention filing.

In their memo, prosecutors further revealed that Routh allegedly sent a letter "several months prior" to his arrest to a civilian witness that stated, "This was an assassination attempt on Donald Trump but I failed you."

The government argued the sole reason Routh was in West Palm Beach on Sept. 15 and the proceeding month was "for one reason and one reason only and that was to kill the former President of the United States."

On the day of the attempted assassination, Trump was playing golf on the course when a Secret Service agent spotted a gun barrel poking out from the tree line near the sixth green, according to investigators.

The agent then fired in the direction of the rifle and saw Routh fleeing the area and entering his nearby vehicle, according to the criminal complaint filed in the case.

In the area of the tree line where the suspect was seen, agents found a digital camera, two bags, including a backpack, and a loaded SKS-style 7.62x39 caliber rifle with a scope, according to the complaint.

Trump was not harmed in the incident and was taken to a safe location by Secret Service agents.

ABC News' Katherine Faulders and Aaron Katersky contributed to this report.