Official investigations into the shooting of former US President Donald Trump at a campaign rally in Pennsylvania’s Butler County on Saturday are under way.

A bullet fired by a sniper from a nearby rooftop grazed Trump’s right ear as he began addressing a crowd of supporters.

Trump, with a bloodied ear, was escorted by US Secret Service agents to a hospital nearby and the shooting suspect, Thomas Matthew Crooks, 20, was shot within seconds by the Secret Service.

As the organisation with responsibility for investigating assassinations and attempted assassinations of presidents and presidential candidates, the Federal Bureau of Investigation (FBI) will take the lead in the probe. However, other agencies including state police, the Secret Service and the Department of Homeland Security will also be involved.

Here is all we know about the ongoing investigation:

Who is investigating the shooting?

The FBI released a statement on Saturday saying that its personnel are in Butler, and are jointly working with the Secret Service to investigate the attempted assassination of the former president.

The FBI is investigating both the shooter and the security that was in place at the rally.

The investigation into the killing of Corey Comperatore, a rally attendee who died while trying to shield his family from bullets, “falls under the State of Pennsylvania jurisdiction, thus the Pennsylvania State Police has investigative responsibility for that murder”, former FBI agent Kenneth Gray told Al Jazeera.

When it comes to investigating the security in place at the rally, the focus is likely to be on security planning before the event and the response to the attack when it occurred, experts said. “The purpose is to identify any gaps in providing security and correcting them for future events,” Gray said.

President Joe Biden has also ordered the Department of Homeland Security to implement an independent review of the incident, Homeland Security Secretary Alejandro Mayorkas said on Monday.

“We are going to really study the event independently and make recommendations to the Secret Service. It is very important that the facts be learned by an independent body outside of the Department of Home Security so that the integrity of the investigative process and the conclusions reached and the recommendations made have the full confidence of the government and the American people.”

What has the investigation uncovered so far?

So far, the FBI has announced only that it has searched the shooter’s home and vehicle, where explosive devices had been discovered. The motive for the shooting remains unknown.

What evidence relating to the shooter is the FBI looking at?

• The most recent FBI update on the case said that bomb technicians had deemed that “suspicious devices” found at Crooks’s home and vehicles were safe. These devices are now being evaluated at the FBI Laboratory.
• Gray said that the FBI investigation would likely “include interviews of witnesses at the rally, anyone that may know the shooter [and] a search of his residence and car.”
• It is also expected that the organisation will carry out “forensic evidence collection, digital forensics and other aspects”, said Gray, adding “Social media will certainly be looked at, as will the shooter’s phone.”

What issues about security have been raised so far?

While the FBI has not revealed details about any part of its investigation, reports emerged on Monday that the rooftop from which Crooks fired at Trump had been “identified by the Secret Service as a potential vulnerability in the days before the event”.

An unnamed former senior Secret Service agent was quoted by US news network NBC saying: “Someone should have been on the roof or securing the building so no one could get on the roof.”

Mayorkas also told ABC News that the shooter should never have been able to get into the position he did: “A direct line of sight like that to the former president should not occur.”

How has the Secret Service responded?

The Secret Service issued a statement on Monday to address rising concerns about the security in place for the Republican National Convention (RNC) in Milwaukee.

Director Kimberly Cheatle said: “I am confident in the security plan our Secret Service RNC coordinator and our partners have put in place, which we have reviewed and strengthened in the wake of Saturday’s shooting.”

On the subject of the shooting at the rally in Pennsylvania, Cheatle said: “Secret Service personnel on the ground moved quickly during the incident, with our counter sniper team neutralising the shooter and our agents implementing protective measures to ensure the safety of former President Donald Trump.

“The Secret Service is working with all involved federal, state and local agencies to understand what happened, how it happened, and how we can prevent an incident like this from ever taking place again. We understand the importance of the independent review announced by President Biden yesterday and will participate fully. We will also work with the appropriate congressional committees on any oversight action.”

What will the investigation into the shooter result in?

So far, the FBI has stated that it does not believe anyone else was involved in the shooting.

“While the investigation to date indicates the shooter acted alone, the FBI continues to conduct logical investigative activity to determine if there were any co-conspirators associated with this attack. At this time, there are no current public safety concerns,” it said in an updated statement on Monday.

As Crooks was shot dead by the Secret Service, “the final result will be an investigation report, as there will not be a criminal case”, said Gray.

If new information is revealed showing that others were involved, however, it is expected that the FBI would expand its investigation.