The gunman who shot at former United States president Donald Trump during an election rally in Pennsylvania on Saturday appears to have acted alone, according to the Federal Bureau of Investigation, which is investigating it as a potential "domestic terrorism" act.

The gunman has been identified as Thomas Matthew Crooks (20).

"At this point in the investigation, it appears that he was a lone actor, but we still have more investigation to go," said Robert Wells, the executive assistant director of the FBI's National Security Branch. 

The FBI, he said, is investigating this as an assassination attempt, and also as a "potential domestic terrorism act."

The counterterrorism division and criminal divisions are working jointly together to determine the motive, he added.

The FBI is leading the investigation into the assassination attempt of Trump at an election rally in Pennsylvania. 

Trump was hit with a bullet that pierced the upper part of his right ear. He is now safe and is continuing with his schedule that includes travelling to Milwaukee to attend the Republican National Convention, which would formally nominate him as the party's presidential nominee against incumbent Joe Biden.

"The shooter may be deceased, but the investigation is very much ongoing. And, because of that, we are limited in what we say at this point," FBI Director Christopher Wray said. 

"What we witnessed yesterday was nothing short of an attack on democracy and our democratic process," he told reporters at a news conference in Washington DC.

According to an FBI official, the investigation into the gunman at Donald Trump's rally Saturday has not yet turned up any mental health issues, threatening posts or other motives, but cautioned it was still early. 

The shooter, as per FBI, used an AR-style rifle chambered in 5.56mm, a common calibre for such weapons.