'Cool it down,' Biden tells nation after Trump assassination bid

President Joe Biden sought to calm a divided nation after his rival Donald Trump survived an assassination bid, saying in a rare Oval Office address Sunday it was time to lower the temperature of America's hostile politics.

"You know, the political record in this country has gotten very heated. It's time to cool it down," Biden said in the televised speech, just the third he has given from behind the historic Resolute Desk during his presidency.

As the country reeled from images of a bloodied Trump waving his fist after a gunman opened fire at a rally in Pennsylvania, Biden added that US politics "must never be a literal battlefield, a, God forbid, a killing field."

The 81-year-old Democrat said that both sides had a responsibility to ease the situation ahead of a deeply polarized election that would now be a "time of testing" for the United States.

The short but forceful speech went without any major hitches -- bar Biden twice referring to the ballot box as a "battle box" -- with the ageing president under close scrutiny following a disastrous debate performance against Trump recently.

Investigators said that they were still probing the motives of 20-year-old gunman Thomas Matthew Crooks, who was shot dead by snipers at the event in Butler, Pennsylvania, on Saturday after firing multiple shots with a legally-bought semi-automatic rifle.

Former president Trump said on social media Sunday that Americans should not allow "evil to win," adding it was "more important than ever that we stand United."

The 78-year-old Republican later added on social media that it was "God alone" who had saved him. Trump's wife Melania called the shooter a "monster."

Security questions

Trump was hit in the ear and left with a bloodied face by the attack, which also killed a bystander while two other people were wounded, but he managed to raise a defiant fist to the crowd as Secret Service agents bundled him away.

He landed later Sunday in Milwaukee for the Republican National Convention, where he will be anointed as the party's presidential nominee, with supporters believing the attack will swing voters behind him in November.

The US Secret Service insisted the agency is "fully prepared" to maintain security at the huge Republican gathering, and that it was not changing its protocols even after the attempt to kill Trump.

But the agency faces searching questions about how the shooter was able to climb onto a rooftop around 150 meters (500 feet) from where Trump was speaking and fire multiple rounds.

Biden said earlier that he had a "short but good conversation" after the attack with Trump.

Investigators are now trying to figure out what drove the shooter, with the FBI saying it believed that Crooks acted alone and had no known ideology.

"We are investigating this as an assassination attempt, but also looking at it as a potential domestic terrorism act," Robert Wells, assistant director of the FBI counterterrorism division, told reporters.

The FBI said the AR-style 556 rifle was believed to have been bought by the shooter's father, but do not yet know how he accessed the weapon or whether he took it without his father's knowledge.

Investigators also found a "suspicious device" in the shooter's car.

'Bullied'

Crooks' former schoolmates described him as a quiet student who often came across as lonely.

"He was quiet but he was just bullied. He was bullied so much," Jason Kohler, who said he attended the same high school as Crooks, told reporters.

Biden meanwhile praised the bystander who was killed, named as firefighter Corey Comperatore, saying he "was protecting his family from the bullets."

The attempt on Trump's life sent shock waves around the world, but the effects on a tight US presidential race in a deeply divided country are uncertain.

US politics have become increasingly hostile, with Trump building his image around inflammatory verbal assaults, and many Democrats expressing fury and disgust at Trump's rise.