Which is scarier, the noise or the silence? Long after the attacks of Sept. 11, the clangor of terror echoes worldwide. But for U.S. investigators, what they don’t hear is almost as frightening as what they do. Terrorist communications, according to Francis X. Taylor, the State Department’s counterterrorism coordinator, have reached levels “probably as high as they were last summer.” Attacks continue. In April, a truck bomb–now thought to be the work of Islamic terrorists with links to al-Qaeda, the network headed by Osama bin Laden–crashed into a synagogue in Djerba, Tunisia, killing 19, including 14 German tourists. On May 8, an apparent suicide bomber in Karachi, Pakistan, pulled his car up beside a military bus loaded with French contract workers, exploded the car and killed 14. Those waiting nervously for a second al-Qaeda attack on the U.S. may have forgotten: it already happened. Last December, shoe bomber Richard Reid tried to blow up an American Airlines plane over the Atlantic in an incident that investigators have long been convinced was an al-Qaeda plot. Though that effort was foiled, the terrorists have not given up. “Just as a wounded animal is the most dangerous of all,” Air Force General Richard Myers, Chairman of the Joint Chiefs of Staff, said last week, “al-Qaeda remains a real threat.” And sometimes a silent one. The investigation into the Sept. 11 attacks is the most comprehensive the world has ever known. Yet after wading through mountains of paper seized in Afghanistan, checking out hundreds of computer discs and interviewing scores of al-Qaeda detainees, investigators have found not a single reference to the Sept. 11 hijackers. “Where’s the intel on these people?” asks a senior FBI official. “Even after all this time, there are no documents and nothing in the humint [human intelligence].” Which raises the spookiest possibility of all: that there could be another al-Qaeda cell out there, just as good–just as quiet–as the one that mounted the attacks on the World Trade Center and the Pentagon. That’s why assessing the capabilities of al-Qaeda now is so important. Figuring out what al-Qaeda can do–and stopping it–requires a mixture of military action and persistent shoe-leather work by cops. Since last fall, 1,600 suspected operatives of al-Qaeda have been arrested in 95 countries. Sometimes you just have to wait. Sources tell TIME, for example, that after years of silence, one of the most mysterious figures in al-Qaeda’s network has started talking to the FBI and a federal grand jury. Ihab Mohamed Ali, known within al-Qaeda by the nom de guerre Nawawi, is an Egyptian-born U.S. citizen who worked with bin Laden’s organization in Sudan and Afghanistan after receiving flight training at the same Oklahoma school where Zacarias Moussaoui, the alleged terrorist who was detained before the Sept. 11 attacks, studied last year. Ali later returned to the U.S. and worked as a cabdriver in Orlando, Fla. He was arrested after the al-Qaeda bombings of U.S. embassies in Africa in 1998 but clammed up. Indicted for perjury, Ali has been detained ever since. If he is talking now, he could shed some much needed light on the early days of al-Qaeda’s international campaigns.