Homeland Security Secretary Alejandro Mayorkas announced an independent review of the shooting and said it will be launched in "days."

"We are, indeed, in the process of selecting who will lead the independent review. The findings indeed will be made public," he said. "It is very important that we achieve transparency so that the American people have confidence in the work of the review and its findings and recommendations. And we need to move with swiftness and urgency, because this is a security imperative."

Mayorkas said he was committed to going "externally of the government" so that "no question of its independence can be raised."

Mayorkas told reporters he has "100% confidence" in Secret Service Director Kimberly Cheatle, and White House press secretary Karine Jean-Pierre said President Joe Biden also has confidence in the director.

Biden told reporters Monday that he has faith in his own Secret Service detail and faith in the agency.

White House press secretary Karine Jean-Pierre declined direct comment when asked about Vance being quick to blame Biden for the attempt on Trump's life.

"I'm not going to politicize this moment. We're not going to politicize this moment. It is wrong to politicize this moment," Jean-Pierre said Monday at her daily White Houe press briefing.

She repeated what Biden said about lowering the temperature and uniting the country when he addressed the nation on Sunday night.

Secret Service protection has been increased for former President Donald Trump, Homeland Security Secretary Alejandro Mayorkas said.

The enhancement is "based on the evolving nature of threats to the former president and his imminent shift from presumptive nominee to nominee," Mayorkas said. "This includes enhancements related to securing the former president during the Republican National Convention this week. I cannot discuss specifics ... as they involve sensitive tactics and procedures. I can say, however, that personnel and other protective resources, technology and capabilities have been added."

"Adjustments" have also been made to President Joe Biden's and Vice President Kamala Harris' protective details, Mayorkas said.

-ABC News' Justin Ryan Gomez

The FBI said it's gained access to suspected gunman Thomas Matthew Crooks' phone and investigators are continuing to analyze his electronic devices.

Searches of Crooks' home and car are complete, the FBI said.

The FBI said it's continuing to investigate the shooting "as an assassination attempt on former President Donald Trump and as potential domestic terrorism."

Nearly 100 law enforcement officials, attendees and other witnesses have been interviewed, the FBI said. "That work continues," the FBI added.