AMSTERDAM, Netherlands – Police arrested eight more suspected Islamic radicals yesterday in the slaying of a Dutch filmmaker who criticized Muslim customs. Lawmakers questioned why authorities hadn’t kept tabs on the alleged killer, who 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 was slain while cycling down an Amsterdam street Tuesday – believed to be the first Islamic terrorist attack in the Netherlands.

Six of the detainees are of Moroccan ancestry, one is Algerian, and the last has dual Spanish-Moroccan nationality, prosecution spokeswoman Dop Kruimel said. The eight are in addition to the suspect arrested minutes after the slaying, a 26-year-old Amsterdam resident of Moroccan origin.

Their ethnic identities raised questions of links to the March 11 train bombings that killed 191 people in Madrid, Spain. 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 September 11, 2001, attacks on America, including many accused of providing logistical support for groups linked to Al Qaeda. Muslim youths are thought to have been recruited here, and experts 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% of the 16 million population. There are about 300,000 Moroccan nationals in the Netherlands.

Mr. 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,” Mr. 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, testing the nation’s famed tolerance and straining already tense relations with the Muslim immigrant population.

Mainstream Dutch Muslim groups condemned the killing. A number of mosques were closed Tuesday night for 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 suspect in the murder of the 47-year-old filmmaker – a distant relative of painter Vincent Van Gogh – 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 himself was not on that watch list. Authorities didn’t release his name, and Dutch press 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 TV reported.

Members of Parliament called for an emergency debate on why the alleged killer – who police say had a record of violent crime – hadn’t 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.