Donald Trump says he doesn’t flat-out blame former president George W. Bush that the Sept. 11 terror attacks happened on his watch. But he can think of three reasons why one could hold Bush responsible.

And, he might add, they are three things a President Trump would do very differently.

“You always have to look to the person at the top,” Trump said Saturday in a telephone interview. “Do I blame George Bush? I only say that he was the president at the time, and you know, you could say the buck stops here.”

So why might one consider Bush responsible?

No. 1: Bush’s immigration policy. “We had very weak immigration laws,” Trump said, adding that perhaps if Bush had had a Trump-style immigration policy, replete with “the strong laws that I’m wanting, these terrorists wouldn’t have been in the country.”

No. 2: People knew that the FBI, the National Security Council and the CIA weren’t sharing information about potential threats. “They were not talking to each other,” Trump said. “If I’m president, I want to have my three most important agencies talking to each other and coordinating with each other.”

And No. 3: George Tenet, Bush’s director of central intelligence, “knew in advance that there would be an attack, and he said that.”

After listing those points, Trump said, “I don’t blame anybody. But it’s possible perhaps something could have been done that was obviously better than the worst attack ever perpetrated on the United States.”

Trump, the real estate mogul, and former Florida governor Jeb Bush have been hurling vitriol at each other since Friday, after Trump pointed out during a Bloomberg News interview that “the World Trade Center came down during [former president Bush’s] reign.”

Jeb Bush retorted on Twitter, calling Trump’s comments “pathetic” and repeating an argument he used during the most recent Republican debate: that his brother “kept us safe.”

Trump returned fire on Twitter, adding that he was just being polite when the subject came up for discussion at the debate. Like it or not, he said, Bush was in the White House when the Twin Towers came down.

“I just didn’t want to embarrass him,” Trump said, insisting that Jeb Bush’s “statement was wrong.”

According to a recent YouGov/Huffington Post poll taken in the days after the last Republican debate, more than 80 percent of Republicans believe George W. Bush did either a “good” or “excellent” job “keeping the U.S. safe while president.”

But even considering Bush’s tenure solely after 9/11, Trump can’t say he agrees.

“I’m not sure that anybody can answer that question. Because you don’t know,” Trump said — effectively making the point that it’s difficult to prove a negative. “Was it something we did? Was it policy that kept it? All I know is that the Twin Towers came down, and I was there.”