MADRID, Spain - Long before Jamal Zougam was picked up as a suspect in the Madrid bombings, he'd flitted across the radar screens of anti-terrorism investigators.

Police knew his apartment - they searched it in 2001. And both he and his half brother, also under arrest, reportedly had been vouched for by an al-Qaida suspect in a monitored phone call.

But as with the Sept. 11, 2001, attacks in the United States, only after death and destruction had been unleashed did such tidbits of intelligence gleaned long ago about the suspects start to jigsaw into a coherent picture.

In Spain, questions already are being asked about whether security agents failed to connect dots that might have enabled them to prevent the terrorists from placing shrapnel-packed bombs on rush-hour trains, killing 201 people.

The investigation into Thursday's attack is focusing on a web of suspected ties to indicted and convicted Islamic radicals that radiate from Zougam, a Moroccan phone salesman. Spanish and Moroccan officials already suspected he was deeply involved in the netherworld of al-Qaida and its offshoots.

Zougam was arrested with his half brother, Mohamed Chaoui, and another Moroccan, Mohamed Bekkali, just two days after Thursday's bombings.

However, both Zougam and Chaoui caught the attention of Spanish anti-terror Judge Baltasar Garzon as early as 2001, according to an Associated Press review of court documents and a French private investigator with access to Garzon's massive dossier on al-Qaida operations in Spain.

But in the fight against terrorists, knowing your enemy doesn't always mean you can stop him from acting. The fruits of globalization - easier travel across borders, quick, cheap and accessible means of communicating - are the same tools terrorists use to slip through cracks.

Mobile phones and e-mail accounts can be used once, then discarded to prevent electronic snooping by intelligence agencies.

Terrorists who need to travel can tap the lucrative black market in forged travel documents.

"They know to create layers and layers of anonymity in the way they communicate," said Magnus Ranstorp, director of the Center for the Study of Terrorism and Political Violence at St. Andrews University in Scotland.

"They are very well versed in staying beneath the intelligence radar screen," he added.

The Madrid bombers appear to have succeeded in keeping their deadly intentions hidden. A U.S. counterterrorism official, speaking on the condition of anonymity, said authorities found no evidence of increased "chatter" - monitored contacts between suspects that might have pointed to a plot - in the days prior to the attack.

While police and intelligence agencies' databases are stuffed with the names of suspects and radicals trained in al-Qaida camps in Afghanistan, investigators can't keep tabs on them all - despite money and manpower that has been poured into beefing up intelligence gathering since the Sept. 11 attacks.

"You cannot expect the police to secure every citizen, every building in every location from every possible terrorist attacks," Interpol chief Ronald Noble said Tuesday at a security conference in Manila, the Philippines. "The only way is to identify terror groups and dismantle them."

Zougam had been identified. Garzon's dossier shows that his Madrid apartment was searched by police on Aug. 10, 2001, turning up a video of mujaheddin fighters in the Dagestan region of Russia and phone numbers of three members of an al-Qaida cell allegedly led by Imad Yarkas. Yarkas is in jail in Spain on suspicion he helped plan the Sept. 11 attacks in the United States.

Moroccan authorities say that Zougam began frequenting radical Islamic circles in 1993 and was recruited four years later by the Spanish al-Qaida cell. But he wasn't among the 35 people, including Yarkas, indicted by Garzon last September for terrorist activities connected to al-Qaida.

Why is not clear. Getting strong evidence to put terrorists away can be painstaking. Sometimes investigators also let suspects stay free to see whether they lead to bigger fish or plots - "a technique that's as old as the Bible," said Glenn Schoen, a Washington-based terror expert.

Chaoui, Zougam's half brother, also turned up in Garzon's probe, in a telephone call monitored in August 2001, according to French investigator Jean-Charles Brisard. Brisard has a copy of Garzon's dossier, which is thousands of pages long, because he is investigating the Sept. 11 attacks for attorneys representing victims.

The call was between Yarkas and another suspected al-Qaida operative indicted by Garzon, Salaheddin Benyaich, Brisard told the AP. Benyaich vouched for the brothers, telling Yarkas they were associates of another alleged member of Yarkas' cell, Said Chedadi.

"These two people are important," Benyaich told Yarkas, according to Brisard.

Spain's massive investigation into the bombings has enlisted the help of U.S. and European police and intelligence agencies and is working to pin down the extent to which the three Moroccans and two Indians also arrested may have been involved. Decades of combatting armed separatists who want a Basque homeland in northern Spain mean police bomb experts here are among the world's best.

But many agree that some time in the future, terrorists will strike again.

"The Madrid attacks are the reminder to the entire world, not just Europe, that these kinds of attacks can occur any day anywhere in the world, and we must try to prepare ourselves the best we can," said Noble, the Interpol chief.