Federal investigators are scouring electronics and examining other evidence as they work to bring more serious charges against the suspect in the apparent assassination attempt of Donald Trump at a Florida golf club, according to law enforcement sources familiar with the matter.

Among the outstanding questions for investigators is whether there is a foreign nexus, including potential links to ongoing Iranian threats against the former president or Ryan Wesley Routh’s previous travel abroad, sources told CNN. So far, there is no evidence indicating the suspect was motivated or instructed by foreign elements to target Trump.

Routh has not cooperated with the FBI since his arrest on Sunday, the sources said, and he currently only faces gun-related charges.

But prosecutors and investigators have made clear that they view the incident as an apparent plot to kill the former president and are working to build a case with those additional charges.

That effort is complicated in part because the US Secret Service has said Routh didn’t have a line of sight to Trump from where he was on the perimeter of the golf club, and that he didn’t fire a shot before a Secret Service officer fired at him and he fled the scene. This means that prosecutors will have to use other evidence to establish his intent that day.

Palm Beach County State Attorney David Aronberg said federal investigators must have clear evidence that Routh intended to target Trump and took a substantial step toward that end if they want to bring the maximum charges against him.

“You don’t have to fire a shot to be charged with attempting to kill a former president or a major presidential candidate,” Aronberg, who is not prosecuting the case but worked with authorities when Routh was first arrested, told CNN. “Those are federal charges that could be in play.”

“But proving it is another matter,” he added, noting investigators are looking for any writings or posts by Routh that show he was targeting Trump which they could use as evidence of express intent.

Routh is set to appear at a detention hearing on Monday when prosecutors will argue he should continue to be held before trial. He has not yet entered a plea, and prosecutors could file additional charges at any time. CNN has reached out to Routh’s defense attorney for comment.

Federal investigators are continuing to review Routh’s devices and other electronics and conduct interviews with potential witnesses, which they hope will provide more clarity around his intentions and motive. Investigators have spoken to family and friends from North Carolina to Hawaii, where they searched a home where Routh lived.

And the FBI is still examining the possibility of a foreign connection given the range of threats against the former president originating from overseas and details about Routh’s travel to other countries, including Ukraine.

Chelsea Walsh, who reported Routh’s name to federal authorities in 2022 after meeting him while working as a nurse in Kyiv and finding his behavior disturbing, told CNN that she called the FBI after the apparent assassination attempt earlier this week.

The FBI followed up on that call with an in-person conversation, Walsh said, though she declined to say what was discussed.

Routh was stopped by Customs and Border Patrol upon returning to the US from Ukraine and referred to Homeland Security Investigations, HSI Executive Associate Director Katrina Berger told lawmakers on Capitol Hill Wednesday.

Berger told a House panel that she believes there would have been no reason to arrest him at the time, but did not say whether any investigation continued.

“Based on the information that I read, there would not have been any reason to immediately take him into custody. He had not made any threats, for instance, against the president or former president Trump,” Berger said.

“HSI responds to port of entry calls – hundreds of responses probably daily. Just because a person isn’t taken into custody immediately doesn’t mean that they’re not subject of a longer-term investigation,” she added.

Concerns about a potential foreign nexus stem, in part, from recent intelligence about Iranian plots to assassinate Trump that have raised concerns about his security before and after a gunman opened fire at the former president’s July 13 rally in Butler, Pennsylvania.

Investigators have not uncovered any evidence, to date, linking Iran or any other foreign government to the suspect arrested on Sunday or the gunman who previously attempted to assassinate Trump at his Butler rally.

But the Iranian threat, in particular, remains a major concern for US officials and has prompted the FBI to seriously investigate the possibility of a foreign link in both cases.

Routh’s social media history

Investigators are also using Routh’s social media history to try to piece together a more fulsome understanding of his intent and motive.

That includes online posts in which Routh put his enmity toward Trump at the center of a rambling and fanciful worldview that fixated on Ukraine, Taiwan, North Korea, and what he called the “end of humanity.”

In June 2020, Routh appeared to say that he had voted for Trump in 2016, but that he had since withdrawn his support of the former president.

“I and the world hoped that president Trump would be different and better than the candidate, but we all were greatly disappointment and it seems you are getting worse and devolving,” he wrote on X, formerly Twitter. “I will be glad when you gone.”

Routh also mentioned Trump in his book, which appears on Amazon without a publisher listed, and is titled “Ukraine’s Unwinnable War: The Fatal Flaw of Democracy, World Abandonment and the Global Citizen-Taiwan, Afghanistan, North Korea and the end of Humanity.”

In that publication, he described the former US president’s withdrawal from the Iran nuclear deal in 2018 as a “tremendous blunder” that drove Tehran closer to Moscow, which it then supplied with drones that have caused devastation across Ukraine.

“You are free to assassinate Trump,” Routh also wrote of Iran in the apparently self-published book.

Those posts, on their own, would likely not be enough to charge Routh with attempting to assassinate Trump, according to Aronberg.

But prosecutors also have the option of charging Routh with other crimes, including aggravated assault with a firearm against a federal law enforcement officer.