Former Florida Gov. Jeb Bush’s campaign is furious with 2016 GOP frontrunner Donald Trump over Trump’s comments that the 9/11 terrorist attacks happened on Bush’s brother former President George W. Bush’s watch. But a review of the basic facts of the situation—and Jeb Bush’s own writings—reveals that even the Bushes admit that “leaky” immigration enforcement was a major driving factor in leading to the terrorist attacks.

While Trump has attacked Bush before—describing the candidate as “low energy”—this line of questioning represents a new level in the war as Trump soars over Bush in recent polling, and Sen. Marco Rubio (R-FL) has essentially replaced Bush as the establishment frontrunner. The latest Trump-Bush bout began in the most recent debate.

In the CNN Republican presidential debate, Trump blamed George W. Bush’s unsuccessful presidency for giving us President Obama, prompting an immediate response from Jeb Bush.

TRUMP: “Your brother – and your brother’s administration gave us Barack Obama, because it was such a disaster, those last three months, that Abraham Lincoln couldn’t have been elected.” BUSH: “You know what? As it relates to my brother, there’s one thing I know for sure. He kept us safe. I don’t know if you remember…”

This exchange sparked a war of words between the two campaigns—culminating in Trump’s declaration that if he were president and were able to enact his immigration policies, 9/11 would not have happened under his watch.

On Fox News Sunday, Trump said:

Look, look, Jeb said, “We were safe with my brother, we were safe.” Well the World Trade Center just fell down. Now am I trying to blame him? I’m not blaming anybody. But the World Trade Center came down. So when he said we were safe, that’s not safe. We lost 3,000 people, it was one of the greatest—probably the greatest catastrophe ever in this country… I am extremely, extremely tough on illegal immigration. I am extremely tough on people coming into this country. I believe if I were running things, I doubt those—I doubt those people would have been in the country. So there is a good chance that those people would not have been in our country With that being said, I’m not blaming George Bush, but I don’t want Jeb Bush to say, “My brother kept us safe,” because September 11th was one of the worst days in the history of this country.

The Bush campaign was quick to attack Trump in response, declaring that “Across the spectrum of foreign policy, Mr. Trump talks about things as though he’s still on ‘The Apprentice’… My brother responded to a crisis, and he did it as you would hope a president would do. He united the country, he organized our country and he kept us safe.”

However, a review Jeb Bush’s 2013 book Immigration Wars reveals that Jeb Bush himself agreed with Trump’s argument and admitted that our “leaky” immigration policy was responsible for the attack.

Bush wrote:

In addition to the Mexican drug cartels, the fact that several of the 9/11 terrorists entered the country lawfully under a leaky immigration system has heightened national security concerns—so much so that immigration enforcement has been placed under the Department of Homeland Security.

Indeed, all of the the nineteen September 11th hijackers were voluntarily imported into the country on visas issued to them by our federal government. Almost all of the visas were issued in the predominantly Muslim country of Saudi Arabia. Four of the September 11th hijackers—Zacarias Moussaoui, Satam al Suqami, Nawaf al Hamzi, and Hani Hanjour—were visa overstays.

In a blockbuster 2002 report, National Review’s Joel Mowbray acquired the visa applications of 15 of the 19 hijackers and exposed how every single one of their applications should have been flatly rejected.

Mowbray wrote:

Brothers Wail and Waleed al-Shehri applied together for travel visas on October 24, 2000. Wail claimed his occupation was “teater,” while his brother wrote “student.” Both listed the name and address of his respective employer or school as simply “South City.” Each also declared a U.S. destination of “Wasantwn.” But what should have further raised a consular officer’s eyebrows is the fact that a student and his nominally employed brother were going to go on a four-to-six-month vacation, paid for by Wail’s “teater” salary, which he presumably would be foregoing while in the United States. Even assuming very frugal accommodations, such a trip for two people would run north of $15,000, yet there is no indication that the consular officer even attempted to determine that Wail in fact had the financial means to fund the planned excursion. They appear to have received their visas the same day they applied.

ABC News covered Mowbray’s report:

Abdulaziz Alomari claimed to be a student but didn’t name a school; claimed to be married but didn’t name a spouse; under nationality and gender, he didn’t list anything. Visa approved. Three months later, Alomari followed his friend Mohamed Atta through airport security… heading for the World Trade Center. Khalid Al Mihdhar, who helped crash the plane into the Pentagon, simply listed “Hotel” as his U.S. destination — no name, no city, no state — but no problem getting a visa.

“They were handing these things out gift-wrapped with ribbons on top,” Mowbray said. “[Al-Qaeda operatives] didn’t have to beat the system, the system was rigged in their favor from the get-go.”

By definition, had the visas been rejected or had the visa-overstays been deported, September 11th would not have happened.

Interestingly, even though George W. Bush remained in office for more than six years after the attack, millions of visa overstays were still not deported. In fact, the United States continues to churn out hundreds of thousands of visas to Muslim immigrants, some of whom have gone on to commit terror attacks in the United States.

As Center for Immigration Studies’ Executive Director Mark Krikorian wrote earlier this year, “This view, that foreign visa applicants rather than the American people are to be served, continues; the number of student visas issued to Saudis, for instance, is up more than 500 percent from 9/11.”

The Republican presidential candidates have taken starkly different positions on the issue of Muslim immigration.

For instance, while Donald Trump has repeatedly articulated a strong stance against increasing the number of Muslim immigrants voluntarily admitted into the country, Marco Rubio has adopted an entirely different position on the matter.

Although four of the 9/11 hijackers were visa overstays, Sen. Rubio authored legislation that would have legalized visa overstays and would have made them American citizens. At the same time, Rubio voted against a visa tracking system offered by his Republican colleague David Vitter, which sought to prevent future foreign nationals from illegally overstaying their visas.

Rubio has also expressed support for expanding the resettlement of Muslim immigrants—on top of the annual 280,000 Muslim migrants the United States admits annually, which includes 100,000 Muslim immigrants who are brought in on green cards and will eventually be able to bring in their family members and vote in U.S. elections.

Moreover, while Trump has declared that he would be cautious about the Muslim migrants that are admitted into the United States on temporary visas, Rubio has introduced legislation that would substantially boost temporary visas to some of the most terror-prone regions of the world. Rubio’s legislation, however, does not include any corresponding enforcement measures to track foreigners brought into the country on temporary visas.

A review of recent terror activity—provided by the Senate Immigration Subcommittee—reveals that a number of attacks have been committed by Muslim immigrants admitted entry to the United States on visas.