Ryan Routh was arrested after a rifle was seen poking through bushes at the West Palm Beach course on 15 September.

A Secret Service agent, who was a hole ahead of Mr Trump, opened fire and the suspect fled but was detained less than an hour later.

Routh, 58, was initially charged with two gun offences but prosecutors have now added to the indictment.

They said Routh left a note, written months before his arrest, signalling his intention to kill Mr Trump.

It was dropped off at the home of an unidentified person who contacted authorities following his arrest, said the US Justice Department.

Prosecutors said it states: "This was an assassination attempt on Donald Trump but I failed you. I tried my best and gave it all the gumption I could muster."

The note was headlined "Dear World" and allegedly offered $150,000 (£111,000) for anyone who could "finish the job".

A handwritten list of dates and locations where Mr Trump was expected to be was also found on him, according to prosecutors.

Routh is said to have spent a month in southern Florida - with mobile phone data showing him near the golf course and Mr Trump's home.

Authorities said six phones, including one showing a Google search of how to travel from Palm Beach to Mexico, were also found in his car.

The golf course incident came two months after Mr Trump survived an attempt on his life while on stage in Pennsylvania.

Routh also faces charges of assaulting a federal officer, possessing a firearm in furtherance of a crime of violence, and the two original firearms charges from last week.

He has not yet entered a plea.

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Prosecutors have also cited a book written by the suspect in which he attacked Mr Trump's foreign policy approach, including in Ukraine - where Routh reportedly travelled in a bid to help the war against Russia.

In the book, he also wrote Iran was "free to assassinate Trump" for having left their nuclear deal.