President Joe Biden on Sunday said he ordered an independent review of security measures after the attempted assassination of former President Donald Trump at a Pennsylvania on Saturday and urged the public not to "make assumptions" about the gunman's motives or affiliations as the FBI investigates.

"I've directed an independent review of the national security at yesterday's rally to assess exactly what happened, and we'll share the results of that independent review with the American people as well," Biden said from the White House, standing alongside Vice President Kamala Harris, Attorney General Merrick Garland and Homeland Security Secretary Alejandro Mayorkas.

"We must unite as one nation," he said. "We must unite as one nation to demonstrate who we are."

The president called for unity in the aftermath of the shooting, saying he had a "short but good conversation" with Trump, his Republican rival, on Saturday night and said he and First Lady Jill Biden were keeping Trump and his family in their prayers. He also said he would again address the nation from the Oval Office at 8 p.m. ET on Sunday.

Biden also memorialized Corey Comperatore, the Pennsylvania man who was killed by the gunman at the rally. Two other individuals were shot and wounded, but were in stable condition on Sunday afternoon, according to Pennsylvania state police.

"He was a father. He was protecting his family from the bullets who were being fired. He lost his life. God love him," Biden said. "We're also playing for the full recovery of those who were injured."

Biden spoke after being briefed by the director of the FBI, Mayorkas, Garland and others in the Situation Room on Sunday morning, saying in brief remarks that while the shooter has been identified, "we don't yet have any information about the motive."

"I urge everyone, everyone, please don't make assumptions about his motives or his affiliations," Biden said. "Let the FBI do their job, and their partner agencies do their job."

He also said he ordered the Secret Service to provide Trump "with every resource capability and protective measure necessary to ensure his continued safety" prior to the shooting and reiterated that directive in the aftermath. And he ordered the Secret Service to review security measures for the Republican National Convention, which is set to begin Monday in Milwaukee. 

Trump himself called for unity and national resilience, and aides said he was in “great spirts and doing well” after being injured during a rally in Butler, Pa. He is pushing ahead with plans to attend this week’s Republican National Convention in Milwaukee, where criticism of Biden and the Democrats is sure to be searing. He said on Sunday he would fly to Milwuakee in the afternoon.

"Based on yesterday's terrible events, I was going to delay my trip to Wisconsin, and the Republican National Convention, by two days," Trump wrote on social media as Biden spoke. "But [I] have just decided that I cannot allow a 'shooter,' or potential assassin, to force change to scheduling, or anything else."

Biden quickly denounced the shooting and talked to Trump in its aftermath. His campaign team, meanwhile, is grappling with how to manage the political implications of an attack on the man Biden hopes to defeat in the November election.

Shortly after the shooting, Biden's reelection campaign said it was putting a hold on "all outbound communications and working to pull down our television ads as quickly as possible.” It was not clear how long the suspension would last.

Biden also cancelled a planned trip to Texas to mark the 60th anniversary of the Civil Rights Act, while Harris postponed a planned campaign trip to Florida on Tuesday, where she was to meet with Republican women voters.

Investigators are still in the early stages of determining what occurred and why. But some Biden critics are calling out the president for telling donors on a private call July 8 that “it’s time to put Trump in the bullseye.”

A person familiar with those remarks said Sunday that the president was trying to make the point that Trump had gotten away with a light public schedule after last month's debate while the president himself faced intense scrutiny from many even within his own party for his dismal performance in the faceoff.

The person spoke on the condition of anonymity to more freely discuss private conversations.

In the donor call, Biden said: “I have one job and that’s to beat Donald Trump ... I’m absolutely certain I’m the best person to be able to do that."

He continued: “So, we’re done talking about the debate. It’s time to put Trump in the bullseye. He’s gotten away with doing nothing for the last 10 days except ride around in his golf cart, bragging about scores he didn’t score … Anyway I won’t get into his golf game.”

In his initial response to the shooting on Saturday night, Biden condemned the attempt on Trump's life.

“Look, there’s no place in America for this kind of violence,” Biden said Saturday night, before hustling back to the White House from Rehoboth Beach, Del., where he had been spending the weekend. “It’s sick. It’s sick. It’s one of the reasons why we have to unite this country. We cannot allow for this to be happening. We cannot be like this. We cannot condone this.”

The Associated Press contributed to this report.