Former US President Donald Trump issued a scathing statement on Monday, tearing into former President George W. Bush over his speech commemorating the 20th anniversary of the September 11 terror attacks where he condemned "violent extremists" - both at home and abroad.

Speaking at a memorial for Flight 93 in Shanksville, Pennsylvania, Bush denounced violent extremism and mourned the loss of brave passengers and crew members on board who tried to take the plane back from the terrorists en route to Washington DC.

"On America’s day of trial and grief, I saw millions of people instinctively grab for a neighbor’s hand and rally to the cause of one another," Bush said. "That is the America I know."

All 40 passengers and crew of Flight 93 died when the plane went down in an empty field as they thwarted the attack on the terrorists' suspected target of either the White House or the US Capitol.

On Monday, Trump wrote in a statement: "So interesting to watch former President Bush, who is responsible for getting us into the quicksand of the Middle East (and then not winning!), as he lectures us that terrorists on the "right" are a bigger problem than those from foreign countries that hate America, and that is pouring into our Country right now."

In his speech, Bush warned of "growing evidence" that dangers to US security are coming "from the violence that gathers within" its borders.

"There is little cultural overlap between violent extremists abroad and violent extremists at home," he said. "But in their disdain for pluralism, in their disregard for human life, in their determination to defile national symbols, they are children of the same foul spirit. And it is our continuing duty to confront them."

"If that is so, why was he willing to spend trillions of dollars and be responsible for the death of perhaps millions of people?" wrote Trump. "He shouldn’t be lecturing us about anything."

Trump criticized Bush's legacy and invoked the twin towers collapse that killed more than 2,600 people that day, reported Daily Mail. "The World Trade Center came down during his watch. Bush led a failed and uninspiring presidency. He shouldn’t be lecturing anybody!" he wrote.

Bush was less than one year into his presidency when 2,977 Americans died in the terrorist attacks. Two planes hit the World Trade Center that day, and one crashed into the Pentagon.  

Bush also issued a statement condemning the January 6 Capitol riot, taking aim at Trump. He said he was "appalled by the reckless behavior of some political leaders since the election and by the lack of respect shown today for our institutions, our traditions, and our law enforcement."

Without mentioning Trump specifically, Bush sent a message: "To those who are disappointed in the results of the election: Our country is more important than the politics of the moment."

"So much of our politics has become a naked appeal to anger, fear, and resentment," he added.

It's not the first time Bush has rebuked his Republican successor.

Trump previously attributed blame to Bush for the 9/11 attacks on the campaign trail in 2016. During a crowded Republican primary debate in February of that year, he told then-candidate Jeb Bush: "The World Trade Center came down during your brother’s reign. Remember that."

"I mean, I know what I'm going to do, but we're not supposed to be talking about it yet," said Trump. "But I think you're going to be happy. Let me put it that way. I think you're going to be very happy."