The Secret Service failed in its sole mission: protecting Donald Trump.

The U.S. Secret Service has only one mission: to protect current and former presidents, but during Saturday’s assassination attempt on Donald Trump, the service botched that mission spectacularly.

The last assassination attempt on a former or current US president was 43 years ago, when President Ronald Reagan was shot in the lung but survived.

Today, American politicians and citizens want to know how an would-be assassin managed to climb to a rooftop vantage point with a rifle and fire four shots at the podium, all in a place that was supposedly already off-limits.

Why were warnings from the public ignored or not acted upon?

And with a tightly-held presidential election just four months away, why hasn’t the Secret Service done a better job of protecting the man many believe will be the next president of the United States?

The investigation, which is already underway, also involves the FBI, the Secret Service and the Department of Homeland Security.

Secret Service Director Kimberly Cheatle has been subpoenaed to testify before a U.S. House of Representatives committee on July 22.

According to the Philadelphia Inquirer, people attending the rally were asked to go through metal detectors to make sure they weren’t bringing weapons.

However, a witness told the BBC that the suspected gunman could be clearly seen crawling around on the roof of a nearby building with a rifle for several minutes.

A witness who called himself Greg said police did not heed warnings he and others gave, suggesting police did not know “what was going on” and that visibility on the roof was poor.

In footage obtained by TMZ, the suspect, named by the FBI as 20-year-old Thomas Matthew Crooks, appears to be seen on camera holding a gun.

One spectator was killed and two seriously injured before the gunman was shot dead. Trump was also wounded in the ear. State police said it was “too early” to determine exactly what happened.

David Dunn, professor of international politics at the University of Birmingham, said Trump’s ducking after the first shots rang out may have saved his life.

Prof Dunn told BBC Radio 5 Live that Trump was “clearly well trained by the Secret Service” in that regard. Before being driven away by agents, Trump stood again and raised his fist in the face of the crowd.

Gregg said he wondered why agents weren’t stationed on nearby roofs and didn’t intervene to remove Trump from the stage after he raised the alarm.

Former Secret Service agent Charles Marino agreed there were questions that needed to be answered, telling the BBC one of the Secret Service’s duties would have been to survey the area and record “areas of concern”.

Experts who spoke to NBC said securing large tracts of land is always a challenge for political rallies and other events. Former Secret Service agent Evi Poumpoulas said there will always be questions: “How do you secure the perimeter? How far do you secure it? Can you cover everything? That’s the question.”

BBCVerify’s analysis shows Crooks was able to get to within just over 130 metres (430 feet) of Trump’s position behind the podium.

Late Saturday it was announced that the FBI had assumed the role of lead investigator in the case, which the bureau is calling an assassination attempt.

One of the special agents said at a press conference that it was “amazing” that the gunman was able to fire his gun before the Secret Service killed him.

But when asked if there had been any security failings, he said his team was “not going to make that assessment” while the investigation was ongoing.

The Secret Service did not attend the briefing, and in an earlier statement confirming the investigation, promised to release more information as it became available.

Meanwhile, the Department of Homeland Security said ensuring the safety of presidential candidates was one of its “highest priorities.”

It is unclear what impact the attack will have on the former president’s future security arrangements, but Trump campaign advisers said it was clear he would need additional security.

A Secret Service spokesman denied the “false allegation” that Trump’s team had requested additional resources, saying the request had been rejected.

In any case, Trump will likely be given the same level of security as a sitting president, former Secret Service agent Joseph LaSorsa told Reuters.

“There will be a thorough review and a major restructuring,” he said. “That should not happen.”

The Republican National Convention will go ahead as scheduled on Monday, with President Trump’s speech due to be held on Thursday, a party statement confirmed.

The event will see Trump formally nominated as the Republican presidential candidate ahead of the November election.