WASHINGTON - When President Obama visits the site of the Sept. 11 terror attacks in New York on Thursday, he will be able to say that with the death of Osama bin Laden, the United States has caught or killed almost everyone allegedly responsible for the carnage.

Now U.S. authorities will aim to capture the last major figure involved in the attacks who remains free, Ayman al-Zawahiri, al-Qaeda's No. 2 leader. Information on computers and other items taken from the compound where bin Laden was hiding, combined with the questioning of current detainees, may help lead them to Zawahiri and new al-Qaeda leaders.

"We have vowed, rightfully so, to track down the people responsible for 9/11," said James Lindsay, the director of studies at the Council on Foreign Relations in New York. "People like Zawahiri are potentially more dangerous in operational terms than bin Laden. He is often referred to as the brains behind al-Qaeda."

The United States is holding seven alleged conspirators in the 2001 attacks on the World Trade Center and the Pentagon at the military base in Guantanamo Bay, Cuba.

Two others were killed in Afghanistan in 2001, according to GlobalSecurity.org, a research group in Alexandria, Va.

Besides Zawahiri, two other alleged conspirators identified by the National Commission on Terrorist Attacks Upon the United States, also known as the 9/11 commission, remain at large.

Zawahiri, an Egyptian surgeon who distributes video messages to bring followers into his radical interpretation of Islam that legitimizes suicide bombers, is believed to be hiding in southwest Pakistan or Afghanistan, said White House intelligence adviser John Brennan in an interview on National Public Radio. The U.S. government is offering a $25 million reward for information leading to his apprehension or conviction.

Zawahiri was indicted in the United States before the Sept. 11 attacks for the August 1998 bombings of U.S. embassies in Tanzania and Kenya that killed 224 people.

Success in tracking down bin Laden doesn't guarantee it will be easier to find Zawahiri, intelligence experts said.

"They're sifting through tons of information that comes across their desks," Lindsay said. "It's like being in a jigsaw-puzzle factory where you're getting a lot of pieces, but you're not sure they go with the puzzle you're working on."

Two other men listed as conspirators by the 9/11 commission, Zakariya Essabar and Mushabib al-Hamlan, are still at large.

"No one can replace bin Laden. He achieved cultlike status," said Rick Nelson, director of the Homeland Security and Counterterrorism Program at the Center for Strategic and International Studies in Washington. Still, many lower-level operatives are likely to try, he said.

Sen. Dianne Feinstein (D., Calif.), chairwoman of the intelligence committee, said more than half of al-Qaeda had essentially been eliminated, though some of its members have been replaced.

"We're hoping to bury the rest of al-Qaeda along with bin Laden," Brennan said at a briefing. "This does not mean that we are putting down our guard as far as al-Qaeda is concerned. It may be a mortally wounded tiger that still has some life in it, and it's dangerous, and we need to keep up the pressure."

A confessed 9/11 mastermind, Khalid Sheikh Mohammed, is awaiting a military trial, along with four of his fellow planners, Walid Bin Attash, Ramzi Binalshibh, Ali Abdul Aziz Ali and Mustafa al-Hawsawi, at the U.S. military base in Guantanamo Bay, Cuba. Two other alleged planners are being held at the base and two were reported killed in Afghanistan, according to GlobalSecurity.org.

The only person tried so far for the attacks was Zacarias Moussaoui, a French citizen, who pleaded guilty to conspiracy and was sentenced to life in prison. Moussaoui said he was part of the plot to fly planes into buildings but was foiled because he was arrested in August 2001 on immigration violations. He then lied to federal officials about the attack plans.