US politicians demanding Secret Service answers after Donald ...

Donald Trump was still receiving treatment from a failed assassination attempt when senior Republican politicians started demanding answers from the US Secret Service. 

Tasked with keeping the former president safe, the elite security agency's failure to prevent Mr Trump being shot in the ear at a public rally has left its senior leaders facing congressional questions. 

There have been no shortage of attacks and threats against US politicians and their families in recent years but an attempted assassination on a presidential candidate under Secret Service protection offers the darkest moment for the agency in decades.

Just hours before his own assassination, president Abraham Lincoln signed the paperwork to establish the Secret Service in 1865.

Initially tasked with combating currency counterfeiting, the Secret Service took on protecting the US president after the assassination of William McKinley in 1901. 

Donald Trump, as a former president, still has Secret Service protection.(AP: Evan Vucci)

Today, the Secret Service maintains permanent protection on the president and vice-president, their families and their predecessors. It also offers temporary protection to high-profile politicians, presidential candidates and visiting heads of state. 

The Secret Service includes both highly-visible bodyguards and advancers that assess threats in before the politician arrives at an event. The bodyguards were the first to storm the stage and cover Mr Trump when the gunfire rang out in western Pennsylvania.

The Associated Press reported that a heavily armed tactical team travels everywhere with a president and presidential candidate and is tasked with confronting would-be attackers.

Pennsylvania state police, at a press conference after the shooting, said the Secret Service was the lead agency tasked with ensuring Mr Trump's safety. It was also responsible for assessing threats in advance.

The Secret Service didn't attend the press conference but in a statement said a suspected shooter fired "multiple shots" at Mr Trump, killing one spectator and critically injuring two others. The Secret Service confirmed it killed the shooter, who the FBI later identified as 20-year-old Thomas Matthew Crook.

The attempted assassination occurred at an outdoor afternoon rally in Butler, Pennsylvania. 

Mr Trump was standing on a stage when he was shot from a nearby rooftop.

Secret Service agents covered Donald Trump after he was shot at a campaign rally.(Reuters: Brendan McDermid)

The AP is reporting that the roof, despite being outside the campaign event, was less than 150 metres from where Mr Trump was speaking.

FBI special agent Kevin Rojek said his agency was working with the Secret Service to determine what precautions the Secret Service had in place before the rally. 

Pennsylvania State Police deputy commissioner George Bivens defended the Secret Service and said it was difficult to secure an open venue. 

"In their defence, what I would want to say is it's incredibly difficult to have a venue open to the public and to secure that against any possible threat, against a very determined attacker," he said.

"The investigation will really give us an opportunity to take a look at where any failures occurred and what can be done better in the future."

Witnesses have told news outlets, including the BBC and NBC, that they saw a man climb onto a roof outside the event. They say they flagged what they saw with police. 

Rally attendees have told media that they had to pass through security checkpoints, including magnetometers, which are metal detectors commonly used to detect handguns. 

The shooter was outside these checkpoints.

It's not the first time a US politician has been shot. Four presidents have been assassinated and others shot.

In recent years, Democrat Gabby Giffords and Republican Steve Scalise both survived shootings. 

In May, a man was sentence to 30 years in prison for attempting to kidnap then-US house speaker Nancy Pelosi and attacking her husband with a hammer. Before that, 13 people were arrested after plotting to kidnap Michigan's governor. 

FBI director Christopher Wray in December told a Senate committee that protecting Americans from terrorism was his agency's top priority. He said the terrorism threat was as complex as ever.

"The greatest terrorism threat to our homeland is posed by lone actors or small cells of individuals who typically radicalise to violence online, and who primarily use easily accessible weapons to attack soft targets," he said.

Mr Trump has had a Secret Service detail since 2015. CNN is reporting his protection was strengthened recently but it remains unclear what prompted that.

The FBI has confirmed there will be a long investigation into what happened, to determine what took place and how the shooter was able to get into position.

That investigation could take months. 

In the meantime, the Secret Service is already facing the prospect of congressional summons to answer politicians' questions.

Secret Service agents swarmed the stage after Donald Trump was shot.(AP: Evan Vucci)

"There are many questions and Americans demand answers," said James Comer, a Republican congressman who chairs a congressional oversight committee.

"I have already contacted the Secret Service for a briefing and am also calling on Secret Service director Kimberly Cheatle to appear for a hearing. The Oversight Committee will send a formal invitation soon."