In a private phone call with Robert F. Kennedy Jr., former President Donald Trump said the bullet "sounded like the world’s largest mosquito."

RFK Jr.'s son posted on X a video of the call, which was filmed from what looks to be RFK Jr.'s hotel room. RFK Jr. had Trump on speaker phone.

The younger Kennedy, Bobby Kennedy III, removed the post after his father said on X that he was "mortified" it became public.

Trump also told RFK Jr. about the phone call he received from President Joe Biden after Saturday’s assassination attempt.

"It was very nice, actually. He called me, and he said, 'How did you choose to move to the right?' [Inaudible] Everybody’s looking straight on," Trump said. "I said, 'I was just showing a chart.'"

-ABC News’ Will McDuffie

The Department of Homeland Security and the FBI have issued a bulletin to law enforcement across the country warning them that, in the wake of Saturday's shooting, violent extremists could try to conduct "follow-on or retaliatory" attacks at events over the next few months related to the 2024 presidential election.

The bulletin urged local, state and federal law enforcement -- and their partners -- to remain vigilant and report any suspicious behavior.

The bulletin also said that "lone actors" or small groups of extremists "likely will continue to view" political rallies and campaign events as "attractive targets."

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Thomas Matthew Crooks, the 20-year-old suspected gunman, planned to attend Robert Morris University this fall, according to a school spokesperson.

Robert Morris University is a private, four-year university located outside of Pittsburgh.

"We have been in touch with law enforcement and stand ready to assist in their investigation," school spokesperson Brian Edwards said.

Crooks had earned his associate degree in science from a local community college and graduated in May with high honors.

He was accepted into both RMU and the University of Pittsburgh, a public-research university also a close drive from his home in Bethel Park, Pennsylvania.

A spokesperson for the University of Pittsburgh said Crooks was told in February that he could attend this fall semester as a transfer student. In March, Crooks informed Pitt that he would not attend.

-ABC News’ Anne Flaherty