Former FBI Director James Comey is being interviewed by the U.S. Secret Service

In this Oct. 21, 2015, file photo, FBI Director James Comey listens while testifying on Capitol Hill in Washington. (AP Photo/Andrew Harnik, File)

Former FBI Director James Comey was interviewed by US Secret Service agents at their Washington, D.C., field office on Friday afternoon, according to law enforcement sources.

Comey was interviewed by agents investigating a social media post he posted Thursday showing shells in the sand on a beach spelling out “86 47,” which has become a popular social media code for removing Trump from the presidency.

Comey was not in custody and appeared voluntarily, a source said.

Trump and fellow Republicans have attacked Comey for the post, demanding an investigation.

Comey “knew exactly what that meant,” Trump said in a Fox News interview. “A child knows what that meant. If you’re the FBI director and you don’t know what that meant, that meant assassination.”

In explaining why he removed the post, Comey wrote on Instagram that he had “posted earlier a picture of some shells I saw today on a beach walk, which I assumed were a political message.”

It was expected that Comey will be asked if he intended the message as a threat, or to inspire others who might consider an act of violence against Trump, the source said. Ultimately, a decision on whether the case is chargeable as a threat against the president may lie with the US attorney in Washington.

“The Secret Service vigorously investigates anything that can be taken as a potential threat against our protectees,” Secret Service spokesman Anthony Guglielmi said in a statement to CNN. “We are aware of the social media posts by the former FBI Director and we take rhetoric like this very seriously. Beyond that, we do not comment on protective intelligence matters.”

Legally speaking, prosecuting Comey may be fruitless as any legal action would stand little chance, especially after a recent Supreme Court decision regarding threats and among a judiciary that has expanded free speech rights in recent years, legal experts say.

“I didn’t realize some folks associate those numbers with violence,” Comey said Thursday of the number “86,” which can refer to something being tossed away or taken out. The corresponding “47” matches Trump’s current term in office as the 47th president. “It never occurred to me but I oppose violence of any kind so I took the post down.”

A Secret Service source familiar with the investigation told CNN that under normal circumstances, such a post wouldn’t warrant a full-fledged investigation but under the current political climate will likely get more attention. Such an investigation, the source said, would serve as a distraction for agents in an organization already stretched thin.