Amsterdam, Netherlands ? Dutch authorities have confirmed that 13 young Muslims arrested on terrorism charges in the Netherlands after the murder of filmmaker Theo van Gogh are members of a radical Islamic group with international links and a Syrian-born spiritual leader.

Dutch intelligence calls the group the “Hofstad Netwerk,” and a Justice Ministry official says 43-year-old Syrian Redouan al-Issar, the alleged spiritual leader, has disappeared without a trace.

Van Gogh was ritually slaughtered on an Amsterdam street Nov. 2, apparently for criticizing Islam. His killing set off a wave of reprisals– attacks on more than 20 Islamic sites in the Netherlands, including a mosque which was gutted by fire early Saturday.

Van Gogh’s alleged killer, 26-year-old Mohammed Bouyeri, of Amsterdam, was arrested in a shootout with police minutes after the filmmaker died of gunshot wounds and a slit throat. Bouyeri had a will in his pocket saying he was prepared to die for Islamic jihad, or holy war.

In the days that followed, the government has come under pressure to release details about Islamic radicals and terrorist recruiting in the Netherlands.

In a letter and notes sent to parliament Thursday, Interior Minister Johan Remkes, who oversees the secret service, gave the clearest picture yet of the Dutch cell allegedly behind Van Gogh’s murder.

Remkes said the Hofstad Network, composed mostly of young Dutch Muslims of North African ancestry, has links to networks in Spain and Belgium; that several members of the group have traveled to Pakistan for training; and that its members were under the influence of al-Issar for many years.

“The number of persons and networks in the Netherlands that thinks and acts in terms of actual violence is, in our opinion, limited,” he wrote. “But the feeding ground from which they spring, is broader … it’s better to think in terms of thousands than hundreds.”

Al-Issar went by several names, including “Abu Kaled,” the Justice Ministry official said. The same name is used by fugitive Muhammad Bahaiah, a courier between Osama bin Laden and European cells.

Al-Issar had sought asylum in Germany beginning in 1995, but has not been seen there since May 2004, the Wall Street Journal reported Saturday.

Remkes said the Dutch secret service realized in spring 2003 that al-Issar was “a leading figure.”