Republican politicians said they would launch swift investigations into how a sniper managed to evade Secret Service agents, climb onto the roof of a building near where .
While information about the incident is still sparse, early media reports said the shooter was outside the security perimeter of the rally venue in Butler, Pennsylvania.
The Federal Bureau of Investigation identified 20-year-old Thomas Matthew Crooks as the "subject involved" in the attempted assassination of Trump.
At least one person interviewed by the BBC said he tried to alert police and the Secret Service about the apparent sniper, to no avail.
Trump supporters criticised the Secret Service, which has the primary responsibility for protecting Trump as a former US president. Billionaire ally Elon Musk called for the agency's leadership to resign.
"How was a sniper with a full rifle kit allowed to bear crawl onto the closest roof to a presidential nominee," asked conservative activist Jack Posobiec on social media site X.
Republican House of Representatives speaker Mike Johnson said on social media that the House will summon the director of the Secret Service and other officials before committees to answer questions.
The Secret Service said shortly after the shooting that it had launched an investigation and briefed Democratic President Joe Biden, Trump's rival in the 5 November election, though the agency did not immediately respond to additional requests for comment regarding its protocols.
The Pennsylvania State Police referred questions to the Secret Service, which did not immediately respond.
Attendee Ben Maser was outside the rally perimeter, listening to Trump, when he noticed two officers seemingly looking for someone. Maser, a 41-year-old welder, started scanning the area too.
"I saw the guy on the roof. I told the officer that he was up there. He went about looking for him," said Maser.
The attack will lead to a review of Trump's security, and he will likely be provided with a level of protection more akin to a sitting president, said Joseph LaSorsa, a former Secret Service agent who served on the presidential detail.
Donald Trump was bundled into a car by Secret Service agents after a shooting incident at a campaign rally. Source: AAP / Gene J. Puskar/AP
The Secret Service said in a statement that it had recently added "protective resources and capabilities" to Trump's security detail, without providing further details.
A retired agent who worked in protective services, speaking on condition of anonymity, said the incident should spark an internal review, and ideally an external one too.
"The gravity of the situation demands thorough scrutiny to prevent such failures in the future and to ensure accountability on all levels," said the former agent.
How are Trump rallies usually secured?
During most of Trump's campaign stops, local police aid the Secret Service in securing the venue. Agents from other agencies within the Department of Homeland Security, such as the Transportation Security Administration, occasionally help.
It is no easy task. Many Trump rallies feature thousands of audience members, take place in the open air and last for hours.
Before the event, agents scan the venue for bombs or other threats, and Trump invariably arrives in a fortified motorcade.
Law enforcement officials typically put up barriers as a perimeter and require all attendees to go through a metal detector to enter the venue. Armed protective agents search all attendees' bags and even wallets. Many rallygoers are patted down by hand.
Secret Service members shielded Donald Trump and assisted him offstage after shots were fired. Source: AAP / Gene J Puskar/AP
"This person either concealed themselves until they became a threat, or were not a threat until they revealed their weapons," said Eckloff.
In the moments after Trump was injured, the former president was quickly surrounded by Secret Service personnel who formed a human shield, while heavily armed agents in body armour and carrying rifles also took to the stage and appeared to scan the area for threats.
Trump was whisked by the agents to a black SUV, and taken to a local hospital, according to the campaign.
