Following the assassination attempt on former United States president Donald Trump during a campaign rally on Saturday, there was much commentary as to whether the US Secret Service had provided adequate security.

The Secret Service is facing scrutiny after a 20-year-old man, identified by the FBI as Thomas Matthew Crooks, who was armed with an AR-style rifle, was able to go unnoticed by security as he scaled a building less than 150 meters away.

Bomb-making materials were found inside Crooks' vehicle and at his home, officials said.

The FBI has taken the lead role in the investigation, the bureau said. On Sunday, the FBI said that the shooter likely acted alone. His motives remain unclear.

Homeland Security Secretary Alejandro Mayorkas, whose department oversees the Secret Service, said that officials were engaged with President Joe Biden's and Trump's campaigns, and were "taking every possible measure to ensure their safety and security".

Bill Pickle, a former deputy assistant Secret Service director, told The Wall Street Journal that how the agency communicated with local law enforcement and used technology such as drones will be investigated.

"The reality is there's just no excuse for the Secret Service to be unable to provide sufficient resources to cover an open rooftop 100 yards away from the site," Pickle said. "And there's no way he should've got those shots off."

House Oversight Committee Chairman James Comer said in a statement that he has "already contacted the Secret Service for a briefing and is also calling on Secret Service Director Kimberly Cheatle to appear for a hearing".

The Secret Service said at a news conference on Sunday that it was confident in its existing security plan and hadn't made any changes following the shooting.

Both Trump and Biden sought calm and unity on Sunday in the wake of the shooting.

Trump arrived in Milwaukee, Wisconsin, on Sunday evening for the Republican National Convention, which began on Monday.

Trump told the Washington Examiner that he had rewritten his speech for the event to focus more on national unity than on Biden's policies. "This is a chance to bring the whole country, even the whole world, together. The speech will be a lot different, a lot different than it would've been two days ago," he said.

Trump is due to receive his party's formal nomination at the convention on Thursday.

In the third time that Biden addressed the shooting, the president said from the Oval Office on Sunday night that it was time to "lower the temperature" in US politics.

"All of us now face a time of testing as the election approaches," he said.

Biden, who is running against Trump in a rematch of 2020, said the two men had a "short but good "conversation on Saturday night.

Crooks, the shooter, was a registered Republican, but had also given $15 to a progressive group on Biden's Inauguration Day more than three years ago, The New York Times reported.

Saturday's shooting has increased calls for independent presidential candidate Robert F. Kennedy Jr. to receive Secret Service protection.

Agencies and Xinhua contributed to this story.

hengweili@chinadailyusa.com