NEW YORK — In the four years since the Sept. 11 attacks, Islamist terrorists have struck repeatedly around the world: Bali, Riyadh, Casablanca, Jakarta, Istanbul, Madrid, London. But not a single attack has occurred in the United States.
Does American intelligence, so routinely paired with "failure" in recent years, earn some credit? Are tighter immigration controls screening out the jihadists?
Was the threat from sleeper cells never as great as it seemed when the twin towers fell? Or is al-Qaida, bloodied but undefeated, patiently plotting the next large attack?
The answer may well be all of the above — a mix of factors not subject to proof, only informed speculation.
"No one really can know for sure," said Richard A. Clarke, the former White House counterterrorism czar in the Clinton and Bush administrations. He, like other experts in the field, emphasizes the absence of a home-grown affiliate of al-Qaida in the United States.
After Sept. 11, 2001, few would have predicted that in 2005, experts would be ruminating about why no additional attacks had taken place. In a Gallup Poll a month after 9/11, 85 percent of Americans said another strike in the United States was somewhat or very likely "over the next several weeks."
No wonder, considering that new threats — some with how-to instructions — seemed to pop up every day: Chemical plants were ticking bombs; crop dusters could spew a deadly trail of microbes; a dirty bomb could make Manhattan uninhabitable.
Before the second anniversary of 9/11, Richard Danzig, a former Navy secretary, said he considered writing a newspaper opinion piece mulling the inaction of al-Qaida in the United States. But the trauma of Sept. 11 was too fresh: "I felt kind of queasy about writing it down," Danzig said.
Now, the absence of an attack seems less like pure luck and more like an achievement. In interviews, a dozen people who track terrorism offered several likely, if partly contradictory, explanations:
A strong offense. All agree that by rousting Osama bin Laden from his Afghanistan base and detaining or killing many top aides, the United States disrupted al-Qaida, which had grown so strong in the 1990s.
Partly as a result, al-Qaida has become decentralized, Clarke said, its goals shared by some 14 affiliated groups around the globe. The major attacks since 2001 have been carried out by home-grown jihadist groups — and unlike Britain or Spain, officials say, the United States does not appear to have found a significant jihadist network.
A strong defense. Tighter immigration controls since 2001 have carried substantial political and social costs, barring innocent foreign students and a few distinguished foreign professors and prominent artists.
But suspicious border guards have also turned away potential terrorists. A Jordanian man denied entry to the United States in 2003 when he landed at O'Hare International Airport in Chicago, for example, is believed to have carried out a suicide bombing 20 months later in Hilla, Iraq, killing 125 people, according to a memo by the Homeland Security Department published last month by The Associated Press.
Terrorism displacement. Many experts believe that the Iraq war, while recruiting many more young Muslim men to the jihadist cause, has also diverted them from U.S. soil by providing a more accessible target: American soldiers in Afghanistan and especially in Iraq.
In the eyes of the jihadists, the troops' presence confirms bin Laden's decade-old claims that the United States is at war with the Muslim world.
An exaggerated threat. After Pearl Harbor, the military ringed the coasts with watchtowers, many of which still stand. But though war raged four years, there was no other surprise attack on the United States.
At least a few scholars believe something similar is happening now, that despite the shock of Sept. 11, the menace to the United States was never as great as supposed. "In the public debate about terrorism, there is almost no one to say the threat is not so great, or that September 11 could have been terrorism's high-water mark," said Benjamin H. Friedman, who wrote a provocative piece in the journal Foreign Policy in July making that case.
Or maybe a big attack is coming after all. Loch Johnson, a University of Georgia political scientist who studies terrorism, points out that eight years passed between the first World Trade Center attack in 1993 and the toppling of the towers in 2001. The quiet was an illusion; the Sept. 11 attacks were being systematically plotted.
"Al-Qaida seems to like major events against the U.S. and has demonstrated a capacity for patience in the planning of such events," Johnson said.
In Iraq, or even in Europe, al-Qaida or like-minded groups may have enough adherents to afford to use them in modest attacks, said Stephen E. Flynn, a homeland security expert at the Council on Foreign Relations. But if only a few dedicated jihadists are in the United States, as is likely, they will be used for maximum impact, he said.
"The terrorists here would be a carefully husbanded resource," he said. "I believe in the U.S., we'll see few attacks, spread farther apart, but catastrophic in nature."
Flynn and others said they fear that the lessons jihadists learn in Iraq — in urban combat, the use of explosives and especially attacks on strategic sites — may one day threaten the United States.
Hurricane Katrina, Flynn said, illustrates the alarming vulnerability of the nation's infrastructure. Had terrorists used explosives to breach the levees in New Orleans, he argued, the same catastrophic flooding would have occurred, and without the warning from the National Weather Service.
Such an attack would have been hard to pull off, but the results would have rivaled the Sept. 11 attacks. "Hundreds dead and $200 billion damage," he said. "Not a bad day's work for a couple of fertilizer bombs."