The Secret Service in recent weeks had increased Donald Trump's security detail due to intelligence indicating there was an Iranian threat to assassinate the former president, according to three officials familiar with the matter.

There is no indication the plot was related to Saturday's shooting.

The sources have suggested Iran has been making these types of threats since the assassination of Iranian military officer Qasem Soleimani in January 2020.

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-ABC News’ Luke Barr, Katherine Faulders, Aaron Katersky and Shannon Kingston

House Oversight Committee Chairman James Comer will issue a subpoena for Secret Service Director Kim Cheatle to ensure she attends the committee's hearing on Monday, July 22, a committee spokeswoman said.

"While the Secret Service confirmed Director Cheatle is expected to appear," Comer will issue a subponea "to head off any attempt by [the Department of Homeland Security] to backtrack on her appearance," the spokeswoman said in a statement. "Director Cheatle must answer to Congress and the American people about the historic failure that occurred on her watch."

In the days since the attack, Cheatle and the Secret Service have faced heightening scrutiny for failing to prevent the incident from happening, and even calls from some to resign. Cheatle said she would not resign from her role.

-ABC News’ Lauren Peller and Will Steakin

Donald Trump Jr. said he was fishing with his daughter on Saturday when his fiancée, Kimberly Guilfoyle, called to tell him that his father was shot.

"Kim calls me [and said], 'Your father was shot,'" Trump Jr. recounted during a fireside chat with Axios.

But she had no other information to give, he said.

"It was 90 minutes before I even knew he was alive," Trump Jr. said.

Trump Jr. said then he was able to speak to his father on the phone.

"Considering the heaviness of that moment, sort of gave me a window for some levity and. you know, I asked him, 'Well, most importantly, like, how's the hair?'" he said.

Trump Jr. said his father responded, "The hair is fine ... a lot of blood in it, but it's fine."

"It was a heavy couple of hours, just that unknown," Trump Jr. said. "And then you finally find everything's OK, and then you sit down, and you start seeing the video when you're watching the news. And it was like, the adrenaline dump leaves, you know. Sort of amazing how calm I was first, and then, it's like, once that goes -- 'poof.'"

Trump Jr. said he believes his father is a changed man after the shooting.

"There are events that change you for a couple minutes and there's events that change you permanently," he said.

He also said he believes that there was a breach in security procedures at Saturday’s rally.

-ABC News’ Lalee Ibssa, Soorin Kim and Kelsey Walsh

President Joe Biden received an updated Homeland Security briefing on Tuesday from national security adviser Jake Sullivan and Homeland Security Adviser Liz Sherwood-Randall, according to the White House.