King Charles wrote a private letter to former President Donald Trump on Sunday, Buckingham Palace said in a statement.

The correspondence was delivered via the U.K. Embassy in Washington, D.C., the palace said on Monday.

"We do not divulge contents of such private correspondence, but safe to surmise that the sentiments are in keeping with those communicated by the PM," the palace said, a reference to Prime Minister Keir Starmer.

The prime minister said Sunday he was "appalled" by the assassination attempt.

-ABC News' Zoe Magee

Former President Donald Trump praised Secret Service agents who acted quickly to protect him and get him offstage following the attempt on his life at his Saturday rally in Pennsylvania.

"They did a fantastic job," he said in an interview with the New York Post on Sunday.

Reflecting on the shooting, Trump said, "I'm not supposed to be here, I'm supposed to be dead."

Trump praised the crowd at his rally for how they reacted in the moment.

"A lot of places, especially soccer games, you hear a single shot, everybody runs. Here there were many shots and they stayed," he said, adding, "I love them. They are such great people."

In the interview, which the Post reported also included a reporter from the Washington Examiner, Trump addressed the call he received from President Joe Biden saying it was "very nice."

Sen. Lindsey Graham, who the Post reported was on the flight with Trump, said of the former president, "He feels like he has a new lease on life."

The FBI says its investigation into the assassination attempt on former Donald Trump remains very active.

An official who spoke to ABC News late Sunday night said that while the evidence currently points toward a lone wolf shooter, the FBI will not be comfortable until they fully exploit the suspect's phone and dig deeper into his life.

In a statement Sunday night, the FBI said in a statement it "continues to conduct logical investigative activity to determine if there were any co-conspirators associated with this attack."

Additionally, the FBI said a motive for the shooting still has not been identified.

-ABC News' Pierre Thomas

A day after being grazed by a bullet in an attempted assassination, former president Donald Trump said he plans to take advantage of the moment and deliver a message of unity to meet the moment.

"The speech I was going to give on Thursday was going to be a humdinger," Trump said as he boarded his flight en route to the Republican National Convention in Milwaukee. "Honestly, it’s going to be a whole different speech now."

"It is a chance to bring the country together. I was given that chance," Trump added.

He said that many people from the political spectrum have called him over the past 24 hours and described the moment he turned his head on stage to look at the screen, which ultimately could have saved his life.

"That reality is just setting in," Trump said. "I rarely look away from the crowd. Had I not done that in that moment, well, we would not be talking today, would we?"

The world has now seen the image of the former president raising his fist with blood on his ear, and Trump said he did that because he wanted the country to know he was going to be OK. He wanted the Pennsylvania crowd to know that "America goes on, we go forward, that we are strong."

Trump recalled, "The energy coming from the people there in that moment, they just stood there; it’s hard to describe what that felt like, but I knew the world was looking. I knew that history would judge this, and I knew I had to let them know we are OK."

-ABC News’ Kelsey Walsh