The Secret Service did not search the perimeter of the golf course where a suspect lurked for nearly 12 hours in the apparent hope of killing Donald Trump, the agency has admitted.

The revelation has increased the pressure on an organisation that was already under intense scrutiny over alleged security failures surrounding a similar attempt on the ex-president’s life two months ago and has raised questions over whether it is adequately funded.

In the latest episode, an apparent assassination attempt was averted after a Secret Service agent opened fire after spotting the barrel of a gun protruding from bushes as Trump played golf with an associate at his club in West Palm Beach, Florida.

A suspect, Ryan Wesley Routh, 58, was later arrested after allegedly fleeing the scene in a black Nissan. He was charged in court on Monday with possession of a firearm as a felon.

The agency said he did not fire a shot in the incident and never had Trump, the Republican presidential nominee, in his sights.

However, amid renewed calls for an increase in Trump’s Secret Service detail, there were questions about how he was able to spend nearly 12 hours undetected in the vicinity of the Trump International Golf Club before an agent spotted the danger.

Routh’s presence in the area immediately before Sunday’s incident has been established from his mobile phone data.

There is also concern over how he knew Trump was likely to be playing golf at the Palm Beach course that day, although it is common knowledge that the former president commonly plays at one of his courses on Sundays.

“The president wasn’t even really supposed to go there,” the Secret Service’s acting director, Ronald Rowe Jr, told journalists, adding that Trump did not have a visit to the course on his official schedule.

He did not clarify whether that meant the agents lacked the time to sweep the course for security risks.

While the agency has won praise for the sharp actions of an individual agent in averting a potential assassination attempt, there are worries over how Routh – who has a long criminal record – was able to enter and remain on the premises.

The Wall Street Journal reported that Americans who encountered him during his time as an activist in Ukraine informed the US authorities because they were concerned about his threats of violence. A nurse who met Routh several times in Ukraine reportedly relayed her concerns about him to a customs and border protection officer at Washington Dulles airport in 2022.

“I’m very concerned at reports that the suspect allegedly was in the bushes for 11 hours,” Beth Celestini, a retired Secret Service agent who protected Barack Obama, told the New York Times. “The Secret Service has protocols where if enacted, this suspect should have been discovered before the incident.”

Ronald Layton, another retired veteran agent, said: “Was this just luck that you caught this guy, or did you have the appropriate mechanisms in place for these kinds of things on the threat spectrum?”

Trump, whose security was stepped up after July’s failed assassination attempt in Butler, Pennsylvania – when a bullet grazed his ear after a gunman opened fire on him at an election rally – called on Monday for added Secret Service protection. “We do need more people on my detail,” he wrote on a discussion about the episode on X.

Rowe has called on Congress to provide additional funding and recently wrote to senators saying that the agency desperately needed more than the $3bn it was allocated for the 2024 financial year.

“The increased mission requirements of the Secret Service necessitate additional resources to ensure that we have the tools, resources and personnel needed to meet these new requirements,” Rowe wrote in a 5 September letter to a Senate sub-committee on Homeland Security.

The plea for more money is being complicated by the looming threat of a government shutdown by the end of this month after House speaker Mike Johnson last week shelved a continuing resolution on spending attached to a bill on election security after conceding it did not have enough support to pass.

Agency officials told congressional staffers in a meeting on Monday that more money was needed for more agents and resources to keep Trump safe in the run-up to November’s election, Punchbowl reported.

Their plea has won support from Joe Biden, who told reporters on Monday that “the [Secret] Service needs more help” and that Congress “should respond to their needs”.