The gunman accused of planning to kill Donald Trump at his Florida golf course was indicted Tuesday on three additional counts, including attempted assassination of a major presidential candidate, the US Department of Justice announced.

Ryan Routh, 58, was arrested on September 15 after Secret Service agents spotted him with a gun near the course where Trump was playing. 

He has already been charged with two gun crimes in relation to the incident. 

The new indictment was issued late Tuesday by a grand jury — a panel of citizens with investigative powers — in Miami, Florida, the justice department said in a statement. 

Court documents indicated that the case was assigned at random to Judge Aileen Cannon — a Trump appointee who stopped criminal proceedings against the former president over his retention of top-secret documents at his private residence.

In addition to attempted assassination of a presidential candidate, the new charges also include possession of a firearm in furtherance of a violent crime and assault on a federal officer, who court documents indicated was a Secret Service agent.

Routh's gun was spotted poking out from the bushes by a Secret Service agent as he scouted ahead during Trump's golf game. The agent fired on the suspect, who fled but was later apprehended. 

A federal judge ruled on Monday that Routh should remain in custody. 

FBI analysis of Routh's phone showed he had been in Florida since August 18, and his devices were located multiple times between that date and September 15 near Trump's golf course and his Mar-a-Lago residence, prosecutors said. 

Before being spotted by the Secret Service agent, he had spent nearly 12 hours in the vicinity of the club, according to his phone location data, the prosecution has said. 

It was the second attempted assassination attempt on Trump this summer. The first took place on July 13 at a rally in Butler, Pennsylvania, when a gunman opened fire, killing one person and wounding Trump in the ear. 

The candidate was otherwise unharmed.