The Federal Bureau of Investigation (FBI) says it is investigating what appears to be a second assassination attempt on US Republican presidential candidate Donald Trump.

The former President was safe after the incident at his golf course in Florida, and a suspect is in custody.

This is what we know.

How it unfolded

The incident played out at about 1:30pm Sunday, local time.

Trump was playing golf at his course in Florida's West Palm Beach when a US Secret Service agent spotted a man with a firearm hiding in the shrubbery surrounding the property, according to Palm Beach County Sheriff Ric Bradshaw.

The agents had been clearing the holes ahead of Trump when they saw a rifle barrel poking through the bushes.

The gunman was about one or two holes away — approximately 400 metres — from where Trump was playing.

Police have said that was "not a long distance" for an AK-47-style rifle fitted with a scope, which was retrieved from the scene.

When the agents saw the muzzle of the rifle, one of them opened fire at the gunman.

He fired at least four rounds, but the man managed to escape, fleeing the scene in a black Nissan car. 

He left behind the rifle, a GoPro, and two backpacks at the scene.

Investigators have not confirmed whether the gunman was able to shoot before fleeing.

Suspect seen fleeing the scene

A witness saw the suspect drive off and was able to take a photo of the car and share it with law enforcement officials. 

The suspect was later stopped driving northbound on Interstate 95 after crossing into Martin County, about a 72-kilometre drive north of Trump's golf course.

He did not identify the suspect or give details on a possible motive.

The man in custody is believed to be named Ryan Wesley Routh, three law enforcement officials told the Associated Press.

The officials identified the suspect to the AP, but spoke on the condition of anonymity because they were not authorised to discuss the ongoing investigation.

The Martin County Sheriff's Office said the man taken into custody was "relatively calm" when detained.

The FBI said it was investigating "what appears to be an attempted assassination".

Dave Aronberg, the state attorney for Palm Beach County, told American news outlet MSNBC that the charges against the suspect would be filed at the federal level.

He also said attempted assassination charges could be considered even if the accused had not fired a shot.

Trump 'safe and well'

Fox News host Sean Hannity, a close Trump friend, said on air that he spoke with both Trump and the former president's golf partner, Steve Witkoff, after the incident.

They told Hannity that they had been on the fifth hole and were about to go up to putt when they heard a "pop pop, pop pop".

Within seconds, he said Mr Witkoff recounted, Secret Service agents "pounced on" Trump and "covered him" to protect him.

Trump was returned safely after the incident to his private Mar-a-Lago club, where he resides in neighbouring Palm Beach, according to a person familiar with the matter.

In an email to supporters, Trump said: "There were gunshots in my vicinity, but before rumors start spiraling out of control, I wanted you to hear this first: I AM SAFE AND WELL!"

"Nothing will slow me down. I will NEVER SURRENDER!" 

The White House said in a statement that President Joe Biden and Vice-President Kamala Harris had been briefed about the incident and were relieved to know that he was safe.

"Violence has no place in America," Ms Harris said in an X social media post.

What security measures were in place?

Trump's campaign confirmed he was safe after the incident but gave no further details.

The former president was injured in an assassination attempt in Pennsylvania on July 13, raising questions about protection for candidates just months ahead of the November US presidential election.

His right ear was grazed and one rally-goer was killed in the gunfire.

The gunman, identified as 20-year-old Thomas Crooks, was shot and killed by a Secret Service sniper.

The shooting in July forced Kimberly Cheatle to resign as Secret Service director under bipartisan congressional pressure.

The Secret Service's new acting director said in August  that he was "ashamed" by the security lapse that led to the assassination attempt.

Trump's team have stepped up security since the assassination attempt in July. 

When he is at Trump Tower in New York, parked garbage trucks form a wall outside the building.

At outdoor rallies, he now speaks from behind an enclosure of bulletproof glass.

The Florida golf course was partially shut down for Trump's game, but there are several areas around the perimeter of the property where golfers are visible from the fence line. 

Secret Service agents and officers in golf carts and on all-terrain vehicles generally secure the area several holes ahead of and behind Trump when he plays. 

Agents also usually bring an armoured vehicle onto the course to quickly shelter Trump should a threat arise.

Police have defended the security presence at the course.

"He's not the sitting president, if he was, we'd have the entire golf course surrounded. Because he's not, security is limited to the areas that the Secret Service deems possible."

The spokesperson added more officers might be deployed the next time Trump was at the golf course, but said the Secret Service did "exactly what they should have done".

ABC/Wires