Several major questions have emerged for the US Secret Service to answer in the aftermath of the shooting of Donald Trump at a rally in Pennsylvania.

The FBI has taken on the role of lead investigator into the incident, during which one person was killed and two others critically injured - while Trump was wounded in the ear.

As the US demands answers, the Secret Service says it is working to discover "what happened, how it happened, and how we can prevent an incident like this from ever taking place again".

Its chief, Kimberly Cheatle, has been summoned to testify before a committee of the US House of Representatives on 22 July.

Here are some of the questions that experts have started asking.

Why was gunman's roof not secured in advance?

It remains unclear how suspected gunman Thomas Matthew Crooks got access to the roof of a building near the rally that was little more than 130m (430 ft) from Trump.

The rooftop was a known vulnerability before the event, according to NBC News, which cited two sources familiar with Secret Service operations.

“Someone should have been on the roof or securing the building so no one could get on the roof,” NBC quoted one of the sources as saying.

As well as the access question, it has been suggested that the line of sight from the rooftop to Trump's podium area should have been blocked off.

Crooks should not have been able to get a direct sight of Trump, Homeland Security Secretary Alejandro Mayorkas told ABC News on Monday.

Mr Mayorkas said officials would "really study the event independently, and make recommendations to the Secret Service and to me".

Were warnings about the gunman passed on?

An eyewitness to the shooting told the BBC that he and others had "clearly" spotted Crooks crawling around on the roof with a rifle. They alerted police but the suspect continuing moving around for several minutes before firing shots and then being shot dead himself, the eyewitness said.

FBI special agent Kevin Rojek admitted it had been "surprising" that the attacker had been able to open fire.

The county sheriff has confirmed that Crooks was spotted by a local police officer, who was unable to stop him in time. Something that remains unclear is whether this information reached the agents around Trump.

Crooks was already on officials' radar, according to a senior law enforcement official. They anonymously told CNN that officers thought he was acting suspiciously near the event magnetometers. This information was allegedly relayed to the Secret Service.

Was Secret Service too reliant on local police?

The gunman fired his shots from what police have described as a "secondary ring", which was patrolled not by the Secret Service but by local and state officers.

A former Secret Service agent said this sort of arrangement only worked when there was a clear plan on what to do when a danger was spotted.

“When you rely on the local law enforcement partners, you better have carefully planned and told them what you expect them to do about a threat,” Jonathan Wackrow told the Washington Post.

The county sheriff has admitted there had been "a failure" but said there was no single party to blame.

Was the event properly resourced?

A former chair of the House Oversight Committee has suggested that the Secret Service was "spread too thin", which compounded the issue that local police would not have been "trained up" to secure an event like Saturday's rally.

Jason Chaffetz, who has previously reported on Secret Service failures, told the Washington Post there was no bigger "threat profile" than for Trump or President Biden, but that this was not reflected in the security presence in Pennsylvania.

The Secret Service has denied suggestions that a request from the Trump team to beef up the staffing had been rebuffed in advance of the rally.

But the Post reported that it had seen an exchange of messages in which a former Secret Service officer asked colleagues how the suspect had got a gun so close to Trump. He was reported to have received the reply: "Resources."

In a statement on Monday, Ms Cheatle said changes had been made to Trump's Secret Service detail ahead of the start of the Republican National Convention, which kicks off in Milwaukee on Monday. She said she was "confident" in the overall plan.

Was Trump taken off stage quickly enough?

The agents who shielded Trump have received praise, including from former agent Robert McDonald, who said they did a "fairly good job" despite there being no exact "playbook" on what to do in such a situation.

But the question has also been asked whether they were fast enough to whisk the former president away into a vehicle.

Footage of the incident shows them rapidly forming a shield around him in the immediate wake of the gunshots, but then appearing to pause as Trump asks to gather his shoes. The former president goes on to pump his fist for supporters.

A Secret Service veteran told the New York Times he would not have waited. "If that's me there, no. We are going, and we are going now," said Jeffrey James.

"If it's me, I'm buying him a new pair of shoes."