Live updates: FBI 'close' to ID on would-be Donald Trump assassin ...

Donald Trump has been seen walking off his aircraft after landing in New Jersey.

Video posted by an aide showed the former president flanked by US Secret Service agents and heavily armed members of the agency's counter assault team.

Trump planned to spend the night at his private golf club in nearby Bedminster, New Jersey.

There was about 120 metres between where the shooter was perched on a roof and the stage where Trump was standing.

Butler County District AttorneyRichard Goldinger has said the shooter was on the roof of the building that was adjacent to the venue.

Authorities have not identified the shooter or their motive.

Police say it's too early to say whether the shooter acted alone.

But police do not believe there is "any other existing threat out there".

Authorities have not identified the victim who was killed or the two who were in critical conditions, but said all three were men.

FBI special agent Kevin Rojek says he doesn't have "specific information" about whether it was a bullet or glass fragments that hit Mr Trump's ear.

The FBI is calling for people with video of the event or information to share it with authorities.

A reporter from Holland asks Lieutenant Colonel George Bivens about how something like this happens.

Reporter: I know it's a philosophical question but how could you talk to the people outside the United States about what happened here?

Bivens: "What I would say is it's a tragic incident that occurred. We are not in a position to start second-guessing how or why or anything else at this point. Just know it will be thoroughly investigated if there is any one to prosecute, that will be done. We will move forward. I don't have a good answer for you."

Lt. Col. George Bivens says it is "incredibly difficult" to secure an open venue, defending the Secret Service response.

"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 says.

"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."

FBI special agent Kevin Rojek was asked if he was surprised that someone was able to fire multiple shots at a former US president.

"We are still working through the security apparatus at the Secret Service had in place, what happened.

"There will be a long investigation into exactly what took place and how the individual was able to get access to location.

"All that is really days, weeks and months of investigation."

A reporter pressed him on whether he was surprised by it.

Here's what he said:

"It is surprising but to get the truth, all the details of that will come out later."

Asked what is the most important facet of the investigation now, Lt. Col. George Bivens says there's no single focus but points back to the shooter.

"If I had to point to where we are right now, the single greatest priority would be to identifying the motive and whether there was anyone else involved," he says.

Lt. Col. George Bivens has apologised for being "cagey", reiterating the need to be "methodical and cautious" with the scene.

"We don't want anyone else getting hurt and candidly there are some concerns (and we) will be able to talk later on about what those concerns are and continue to be," he says. 

Pennsylvania State Police Deputy Commissioner of Operations, Lieutenant Colonel George Bivens says police won't be identifying the other victims yet.

"We have notified a number of family members but they have not had an opportunity to notify extended families."

Earlier, we reported one audience member had died and two others were in a critical condition.

Mr Bivens said all three victims were men.

FBI special agent Kevin Rojek says he doesn't have knowledge of whether Trump was hit with a bullet or glass fragments. 

There have been reports of both, but Trump says it was a bullet that pierced his upper right ear.

"We don't have that specific information as far as the injuries to the former president," he says.

FBI special agent Kevin Rojek says authorities could reveal the shooter's identity soon.

But he says he wants to be "100 per cent" sure before the shooter is named.

"We should be able to do that within the next few hours, I imagine," he says.

Pennsylvania State Police comissioner Chris Paris says it's "too early" to say if it was a lone wolf attack.

"It will be some time until we can conclusively say or answer that question," he says.

"I am aware of that law enforcement had responded to a number of reports of suspicious activity." 

Pennsylvania State Police Deputy Commissioner of Operations, Lieutenant Colonel George Bivens says:

"At this time, we have no reason to believe that there is any other existing threat out there.

"We are doing everything we can to make sure that this is thoroughly investigated.

"If there is any information developed that anyone else was involved, that between [police] and the FBI, that will very quickly be followed up on."

Pennsylvania State Police Deputy Commissioner of Operations, Lieutenant Colonel George Bivens says shots were heard shortly after 6pm this evening.

"It was a chaotic scene," he says.

"Law enforcement acted heroically, quickly identifying and and neutralising the threat, as well as responding to assist the various victims."

He adds the alleged crimes fall under both state and federal juristriction.

FBI special agent Kevin Rojek declines to identify the shooter and their motive.

"At this time, we are not prepared to identify who the shooter is.

"We are close to that identification.

"And as soon as we are 100 per cent confident in who that individual is, we will share it with the press.

"With that being said, also, we do not currently have an identified motive, although our investigators are working tirelessly to attempt to identify what that motive was."

Special agent in charge of the Pittsburgh FBI Kevin Rojek says field FBI agents are "working feverishly to attempt to identify the individual who did this."

"Right now, we need the public's help," he says.

"Anyone who was on scene who saw anything, who identified any information, please report that to the FBI."

Kevin Rojek, who is in charge of the FBI Pittsburgh field office, is speaking now.

"This evening we had what we're calling an assassination attempt against our former president, Donald Trump.

"It's still an active crime scene."