A source briefed on the intelligence confirms the contents of a Joint Intelligence Bulletin put out Monday by the FBI and the Department Homeland Security warning that extremists may carry out attacks they perceive to be in retaliation for the attempted assassination of Donald Trump.

“The FBI and DHS remain concerned about the potential for follow-on or retaliatory acts of violence following this attack, particularly given that individuals in some online communities have threatened, encouraged, or referenced acts of violence in response to the attempted assassination,” the bulletin said, according to the source.

The source said the bulletin warns that election-related infrastructure is believed to be under heightened threat of attack. No specific targets were mentioned.

The four-page bulletin was first reported by Politico. It comes after FBI Deputy Director Paul Abbate told reporters Sunday the FBI is seeing increased violence rhetoric online.

“We are seeing, although the rhetoric regarding threats of violence has already increased online, we’re seeing that tick up,” he said.

Secret Service Director Kimberly A. Cheatle acknowledged that "the buck stops with me" and that the assassination attempt on former President Donald Trump "should have never happened" in an interview with ABC's "Good Morning America" that aired today.

"I’m being told that the shooter was actually identified as a potential threat of suspicion. Units started responding to seek that individual out. Unfortunately, with the rapid succession of how things unfolded, by the time that individual was eventually located, they were on the rooftop and were able to fire off at the former president," Cheatle said.

She said the building the shooter fired from had a sloped roof, making the securing of it challenging, so a decision was made to put people inside the building. Secret Service was tasked with securing that inner perimeter, while local law enforcement was responsible for the outer perimeter, she said.

But Cheatle took full responsibility for the horrifying incident. "I am the director of the Secret Service. It was unacceptable, and it’s something that shouldn’t happen again," she said.

Warm tributes have been paid to Corey D. Comperatore, 50, who was shot and killed at the rally where former President Trump narrowly survived an assassination attempt on Saturday.

An online obituary posted by a local funeral home painted a portrait of a loving and much-loved family man. Born in 1974, he went to Freeport High School and worked as a tooling engineer.

He spent 10 years in the U.S. Army Reserves and was chief of the Buffalo Volunteer Fire Department in the early 2000s.

He is survived by his wife, Helen, and their two daughters, Allyson and Kaylee.

"His courage was not the loud and boisterous kind; it was the courage of quiet resilience, the strength to be vulnerable, and the bravery to lead with love," the obituary said.

"Corey’s legacy is not just in the major milestones he achieved but in the small acts of kindness that marked his everyday life," it continued.

His funeral will be held in private but friends will be able to pay their respects from 2-4 and 6-8 pm at Laube Hall in Freeport.