AMSTERDAM -- Police arrested eight more suspected Islamic radicals yesterday in the slaying of a Dutch filmmaker who criticized Muslim customs, but lawmakers questioned why authorities had not kept tabs on the alleged killer, who police say had a record of violent crime and contacts with a group under surveillance.
The arrests were made in the 24 hours since Theo van Gogh, 47, was slain while cycling on an Amsterdam street Tuesday.
Six of the detainees are of Moroccan ancestry, one is Algerian, and one has dual Spanish-Moroccan nationality, prosecution spokeswoman Dop Kruimel said. In addition, the shooting suspect, a 26-year-old Amsterdam resident of Moroccan origin, was arrested minutes after the slaying.
The detainees' ethnic identities raised questions of links to the March 11 train bombings that killed 191 people in Madrid. Twenty-nine suspects, mostly Moroccans, have been charged in those attacks; others arrested were of Algerian, Spanish, Tunisian, and Egyptian origin.
The Netherlands has arrested more than 40 terrorism suspects since the Sept. 11, 2001, attacks on the United States, including many accused of providing logistical support for groups linked to Al Qaeda.
Muslim youths are thought to have been recruited here, and specialists say they believe a number of cells in the Netherlands provide funding to foreign terrorist cells.
The country is home to 3 million first- or second-generation immigrants, almost 20 percent of the 16 million population. There are about 300,000 Moroccan nationals in the Netherlands.
Kruimel said five of the suspects, whose identities were not released, were detained and released during an October 2003 investigation into a potential terrorist threat.
''They were previously known to us," Kruimel said. ''As of now, only one suspect is being held for Van Gogh's murder, but the investigation will determine if others may have been connected."
The Dutch have reacted with outrage to the killing of the filmmaker, a relative of painter Vincent van Gogh, straining tense relations with the Muslim immigrant population.
Mainstream Dutch Muslim groups condemned the killing. A number of mosques were closed Tuesday night because of fear of vandalism, and political figures were given additional police protection.
Newspaper NRC Handelsblad reported that conservative politician Ayan Hirsi Ali, who wrote the script for Van Gogh's latest provocative movie criticizing the treatment of women under Islam, received a death threat in an e-mail yesterday that read ''You're next."
The Justice Ministry said the murder suspect is a Muslim radical associated with Islamic fundamentalists on a terrorist watch list.
Interior Minister Johan Remkes confirmed that the suspect was known to have associated with a group of 150 radicals who are watched day and night by the Dutch secret service for fear they may commit a terrorist act. The suspect was not on that watch list. Authorities did not release his name, and Dutch media identified him only as Mohammed B.
The suspect had contacts with Samir Azzouz, an 18-year-old Moroccan immigrant accused of plotting terrorist attacks against Dutch targets, NOS Dutch television reported.
Members of parliament called for an emergency debate on why the alleged killer, who police say had a record of violent crime, had not been stopped.
''Is this a murder, or is this a terrorist attack?" said Jozias van Aartsen, leader of the conservative VVD party. ''The facts must come out very, very quickly."
Van Gogh released a fictional film in August about the mistreatment of Muslim women. In the film, women were shown naked with texts from the Koran scrawled on their bodies.
Police and eyewitnesses said the attacker shot Van Gogh, stabbed him, cut his throat with one knife, and pinned a note to his chest with another.
The note is said to have contained texts from the Koran in Arabic, though police would not confirm this.
According to NRC Handelsblad, the note called for an Islamic holy war, or jihad.
Van Gogh's killing stirred outrage and fears that Dutch people will no longer feel free to speak their minds.
Immigration minister Rita Verdonk told 20,000 Dutch who flocked to Amsterdam's central square for a wake Tuesday night that ''we won't take this."
Verdonk called an emergency meeting with leaders of Muslim groups to discuss how to avoid confrontations, and Prime Minister Jan Peter Balkenende appealed for calm.