WASHINGTON, March 1 ? Of all the milestones in the Bush administration's 18-month campaign against terrorism, the apprehension of Khalid Shaikh Mohammed, possibly the most fearsome of Osama bin Laden's chief lieutenants, came at a critical juncture.


President Bush's critics have been complaining that his focus on President Saddam Hussein had distracted the nation from the war against Al Qaeda. The steady movement toward a war in Iraq had only seemed to escalate the risk of another terrorist attack.

But Mr. Mohammed's arrest was a heavy blow to Al Qaeda and good news for the United States, when that has been a scarce commodity.

Intelligence officials said today that Mr. Mohammed had represented a grave threat to the United States. The officials, some of whom suspect that his terrorist activities began with the first World Trade Center bombing in 1993, said his planning for further operations had continued after the Sept. 11 attacks and continued to grow after the military smashed Al Qaeda's base of operations inside Afghanistan last year.

Since then, officials said he had attempted to infiltrate Qaeda operatives into the United States and had been behind an effort by a Qaeda recruit, Jose Padilla, to begin work on a radiological bomb that uses conventional explosives to spew radioactive material in the air. Mr. Padilla was arrested in May 2002.

Moreover, Mr. Mohammed had tried to organize terror plots in Europe and South Asia, the officials said, and played a significant role in trying to build Al Qaeda's expertise in chemical and biological weapons.

"Other than bin Laden, there is practically no one we would have liked better to have in custody," a senior American intelligence official said today. "It's pretty damn significant."

Though Osama bin Laden is still elusive, and his second in command, Dr. Ayman al-Zawahiri, is also a prized target, Mr. Mohammed was viewed both as the "mastermind" of Sept. 11 and as Al Qaeda's most skilled operational planner.

"He holds the ignition keys," said Magnus Ranstorp, deputy director of the Center for the Study of Terrorism and Political Violence at St. Andrews University in Scotland. "He could tell what is being planned, on what scale, who is involved and where they are. This is a huge blow to Al Qaeda, because Shaikh Mohammed is the contact man for operatives all around the world."

More broadly, Mr. Mohammed's capture came at a critical juncture in the campaign against terrorism when fears of a catastrophic attack remained very high, racheted up by the possibility of war with Iraq even as the country's alert level was lowered ? a decision that seemed to be made to allow the government to increase the threat level again in the event of hostilities.

It came at a time when the Bush administration, seemingly intent on hostilities with Iraq, faced increasingly determined international opposition from some of its oldest allies, who are trying to block its agenda at the Security Council. Today, even Turkey seemed to flinch rather than allow the United States to use its territory for war.

Mr. Mohammed's arrest suggested that American counterterrorism agents were capable of significant direct action after months when the government's security apparatus seemed caught in the throes of reorganization.

It was not until today that the Department of Homeland Security officially came into existence and even now it seems to be struggling to define its purpose and role.

With Mr. Mohammed transferred to American custody after his capture, in an arrest carried out by Pakistani authorities and guided by C.I.A. officers with the assistance of the F.B.I., intelligence officials expressed hope that Mr. Mohammed's cooperation might yield more valuable insights into Al Qaeda than any other detainee has provided so far.

The officials said he could describe the organization, financing and planning of the Sept. 11 hijackings. Moreover, because he was Al Qaeda's chief operations officer, he could also explain the terror network's current plans for attacks. He could even provide fresh clues about the whereabouts of Mr. bin Laden himself.

In addition, he may be able to provide more details about the operational abilities of Al Qaeda. Intelligence officials have said that the terror network was badly disrupted by the war in Afghanistan, but it has never been clear how much Mr. bin Laden has been able to rebuild his ability to carry out terror operations.

Intelligence officials regard Mr. Mohammed as a far bigger catch that the other two major Qaeda figures captured so far, Abu Zubaydah and Ramzi bin al-Shibh. Some of their information has been significant, but officials continue to debate how much of it is true or meant to mislead their interrogators.

Terrorism investigators said today that Mr. Mohammed, as top operational leader with long experience, is likely to have more important information that could lead to the disruption of future attacks.

"We think this guy knows a lot about what they are going to do," one counterterrorism official said.

Mr. Mohammed's arrest seemed to make him a likely candidate for a military tribunal, although no one in the government seemed certain about what legal procedures might be used in his case.

Other senior Qaeda operatives who have been arrested have remained in American custody at undisclosed locations overseas, and questioned under conditions and legal standards that have never been made clear.