The arrest in Serbia (search) of a top terrorist fugitive has raised fresh concerns of an Al Qaeda (search) presence in the volatile Balkans (search), where thousands of U.S. and other international troops are stationed as peacekeepers.

Abdelmajid Bouchar, a 22-year-old Moroccan, sought for involvement in last year's train bombings in the Spanish capital Madrid, that killed nearly 200 people, was caught at the Belgrade railway station in June.

The arrest, revealed earlier this month, revived concerns that the Balkans — with its porous borders, unsophisticated security systems, rampant corruption and organized crime — could serve as a haven for Al Qaeda-linked terrorist groups.

Local officials and experts have long warned that the Balkans at least is a major transit route for the terrorists, as well as for organized crime, including human and drug trafficking. They said the two often go hand in hand.

Serbian Interior Minister Dragan Jocic said police believed Bouchar was most likely passing through Serbia. He noted that "Serbia-Montenegro lies on important east-west transit routes."

Bouchar was arrested by chance during a routine police patrol check at a train that arrived to the Serbian capital from the northern town of Subotica, located on the border with Hungary, Serb authorities said.

Bouchar was sitting in a train compartment with several other people. He said he was an immigrant from Iraq en route to Western Europe — a common sight for Serbia's police which are used to escorting people who are heading west.

But Bouchar stood out, they said. He was traveling in the wrong direction, from north to south, had no documents on him and was too well-dressed for a poor Iraqi immigrant in search of a better life in Western Europe.

A month and a half later, after weeks of back-and-forth with Interpol, it turned out that Bouchar was one of the world's most wanted fugitives.

"He was arrested thanks to the good thinking of a police officer," said Darko Trifunovic, who teaches at Belgrade's security faculty. "This wasn't a well-planned action."

No details about Bouchar's stay in Serbia have been made public. Jocic told The Associated Press that an investigation was under way to determine what he was doing in Belgrade and whether he had any associates here.

Zoran Dragisic, a terrorism expert from Belgrade's Faculty of Defense, warned that the Balkans could be more than just a transit station.

"The Balkans is the springboard for Europe-bound terrorism," he told AP. "We should all be extremely careful."

Dragisic claimed that Al Qaeda put down roots in the Balkans in the early 1990s, when the region exploded in a series of ethnic conflicts. The political turmoil and ensuing instability led to the collapse of the security network, allowing organized crime to flourish.

News reports during the conflict in Bosnia suggested that outsiders joined Bosnia's Muslims in their conflict with the region's Serbs and Croats — though the extent of their impact in the chaos was never clear. Dragisic said that radical Islamic fighters came to the region to fight.

Some of the outsiders married local women and stayed long after the end of the 3 1/2 year war.

In 2002, during worldwide anti-terrorist raids following the Sept. 11, 2001 attacks in New York, six Arab men suspected of ties with Al Qaeda were arrested in Bosnia and shipped to the U.S. base in Guantanamo Bay, Cuba. The United States suspected them of planning attacks against foreign embassies in Bosnia.

Dragisic argued that Balkans is "convenient" for the terrorist groups and criminals because "you can buy anything, including your freedom, here with a couple of thousand euros."

"The states here are weak and corrupt," he said. "You can do anything here."

International officials in Bosnia and Kosovo — which both have large Muslim population and foreign troops deployed as peacekeepers — say they have no evidence of Al Qaeda presence, but are closely monitoring the situation.

"One of our major tasks in Bosnia is preventing terrorism," said NATO's spokesman in Sarajevo, Maj. Dwight Mood. "We are constantly monitoring to make sure the seed of terrorism is not planted here in Bosnia."

Lt. Col. Bridget Rose, spokeswoman for Bosnia's European Union peace force, acknowledged that "terrorism is a global threat and a global problem and all our efforts to bare down on organized crime and corruption have an element of concern about terrorism."

In Kosovo, Col. Charles de Kersabiec, a NATO spokesman, said that "from the military point of view, there is no specific threat from Al Qaeda."

Serbia and other Balkan countries so far haven't been targeted in terrorist attacks similar to those that hit Spain, Great Britain, the United States or their allies in the Islamic world. Local Serbian officials downplayed the terrorism threat even after Bouchar's arrest.

Interior Minister Jocic said that the "arrest showed Serbia's resolve to deal with terrorism and organized crime."

"Serbia is part of a European front against terrorism," he added.

Another official, Serbia-Montenegro's Human Rights Minister Rasim Ljajic said: "I don't think Serbia-Montenegro is in danger, we are not interesting to them (the militants)."

Still, Ljajic added that "we have to be part of global anti-terrorism network, but we should be careful not to draw the rage against us."

But expert Dragisic warns: "We must not fool ourselves that we are not the target."

"This region is extremely threatened," he said.