On Sept. 13, 2001, the United States imposed a nationwide no-fly zone, yet more than 140 people were allowed to leave the country.

	"House of Bush, House of Saud: The Secret Relationship Between the World's Two Most Powerful Dynasties" By Craig Unger Scribner ($26)

Nearly all of them were Saudi, and roughly two dozen were kin to the man who had just orchestrated the Sept. 11 attacks: Osama bin Laden.

What kind of intelligence failure allowed this to happen, and who allowed these people to leave?

Given that 15 of the 19 hijackers were Saudi, what was the rush in squandering what might have been a potential intelligence mother lode?

Craig Unger first reported this story in Vanity Fair magazine. In his new book, he places this incredible scenario in the context of a decades-old relationship between the ruling families of Saudi Arabia and America.

In a year when President George W. Bush will campaign as tough on terror and strong on homeland security, Unger's book is essential reading.

Not only does it pose disturbing questions about Saudi involvement in 9/11 -- witting or unwitting -- it presents a frighteningly believable case that the Bush administration's cozy relationship with the royal house of Saud precipitated this catastrophe.

Unger begins in the late 1950s and early 1960s, when George H.W. Bush was an oil man in Texas whose early success included drilling the first offshore well for Kuwait.

Bush left the oil business in 1966 to get into politics and eventually became director of the CIA under President Ford, just as Saudi businessmen close to the royal family -- including the head of Saudi Arabia's most corrupt bank -- began investing in Texas banks and real estate.

But this is just the beginning of the relationship. During the 1980s, as petrodollars flowed into Saudi Arabia, the Middle Eastern country became a convenient money-launderer for weapons' purchases as the Reagan administration covertly supported right-wing guerrilla operations.

The United States sent money to the Contras in Nicaragua through Saudi Arabia. During the Iran-Iraq war, the U.S. supported Saddam Hussein, whom the CIA had first hired as a 22-year-old assassin in 1959, with weapons passed through Saudi Arabia.

U.S. allegiance with the Saudis had proved so convenient that it was again used to prop up the militant Mujahideen in Afghanistan in its fight against the onetime Soviet Union's occupying army.

Saudis matched American support dollar for dollar, and a scion of the family closest to the Sauds was sent to Afghanistan to build roads: Osama bin Laden.

The Saudi rulers gained extraordinary credibility among their country's poor by supporting bin Laden through a network of Muslim charities.

They also funded madras schools that taught militant extremism. They failed, however, to provide jobs for the graduates.

The result was a growing band of young male recruits for bin Laden's al-Qaida movement.

The Bushes received personal help from their Saudi friends. When oil prices were dropping in the 1980s, an investor close to the royal family bailed out a tiny Texas oil company called Harken Energy. One of its directors was George W. Bush.

The real payoff began when Bush senior joined the Carlyle Group, an investment firm that used its contacts in the government to secure lucrative contracts, including enormous ones in Saudi Arabia, and then sell them at a high profit.

In addition to Bush senior, the Carlyle group once had the younger Bush on its board and former Reagan and Bush aides as consultants. Saudis also invested directly in Carlyle funds. Eventually, Carlyle became the pipeline for $1.4 billion to flow from the House of Saud to the Bush family and its interests.

So how much influence does this $1.4 billion buy? Unger makes a compelling case that it kept our eyes off the rising extremism in Saudi Arabia.

In his 2000 campaign, Bush courted the Arab-American vote by bringing up the need to stop the use of secret evidence in detaining Arab-Americans, as well as racial profiling at airports.

It worked. According to Unger, 55,000 Arab-Americans in Florida voted for Bush. Bush's margin of victory, remember, was allegedly 500 votes. As Unger puts it, "Without the mobilization of the Saudi-funded Islamic groups, George W. Bush would not be president today."

The Bush administration was not just soft on terrorism before Sept. 11, but also was asleep, writes Unger.

What was the administration focusing on at this time? It was busy repairing a Saudi-American split over a remark Bush had made criticizing Palestine. Bush immediately reversed his position, but his reward was short-lived.

In fact, it didn't even survive 9/11, when the world's good-will toward America was at its highest. Saudi officials refused to freeze bank accounts, and they refused to allow Americans to use their soil during the invasion of Afghanistan.

Here's a final bombshell -- borrowed by Unger from Gerald Posner's book "Why America Slept:"

Prince Ahmed bin Sultan, a member of the Saudi royal family, reportedly knew about the Sept. 11 attack in advance but was among other Saudi citizens spirited out of the United States in the secret airlift following the attack.

It seems likely that Unger's work will be labeled conspiracy theory, but his research is too cautious, too elemental to support that claim.

The book builds a momentum of discovery that makes it impossible to stop reading. Will the American people hear this message during the presidential election? As for Unger -- to borrow a line from comedian Billy Crystal at the Oscars -- he'd better be preparing for his income tax audit.