DONALD Trump has vowed to reopen the original investigation into the 9/11 terror attacks 15 years after the massacre shook the world.

Disenfranchised working class voters propelled the billionaire reality star into the White House this week in one of the biggest election upsets in US history.

In his first days as President-elect, Republican Trump has given few clues on how he plans to honour his pledge to "Make America Great Again".

But ahead of his Brexit-style election earthquake, Trump threatened to lift the lid on one of the most catastrophic terror attacks in modern history.

Speaking at a Republican rally in Henrico County, Virginia, Trump pledged to launch a fresh probe into the 2001 September 11 attacks.

He claimed that the attack by Islamic terrorist group al-Qaida was not properly investigated and vowed to get to the bottom of it.

Pressed about the attacks on the World Trade Center, and his plan to prevent further massacres, he said: "First of all, the original 9/11 investigation is a total mess and has to be reopened.

Speculating over the collapse of the skyscraper after the attack, he said: "How do two planes take out three buildings in the same day?

"I never got my head around the fact that nothing is mentioned about the destruction of Building 7 in the 585-page document."

Accusing "19 high-jackers from Saudi Arabia" of carrying out the attack, he blasted George Bush's administration for not taking legal action against the country in the February debate.

He added: "The World Trade Center came down during the reign of George Bush.

"He kept us safe? That is not safe. That is not safe.