THE Dutch secret service has been infiltrated by an Islamic extremist linked to the killer of Theo van Gogh, the Dutch film-maker whose murder has accelerated Holland’s transformation from one of Europe’s most tolerant countries into a society increasingly polarised by fears about immigration.

Intelligence sources said last week that a mole working for a terrorist group codenamed the “Hofstad cell” had been arrested on suspicion of relaying information collected by the authorities to its members.

The disclosure has compounded the embarrassment of the security services, which have admitted they were watching Mohammed Bouyeri, the 26-year-old Dutch-born Moroccan charged with van Gogh’s murder, from August 2002.

They ended their surveillance less than two weeks before Bouyeri allegedly shot the film director six times as he cycled down an Amsterdam street, then butchered him with a knife.

The murder — apparently provoked by the 47-year-old van Gogh’s outspoken attacks on radical Islam, including a film depicting Koranic verses on a naked female back to represent the supposed oppression of women — has been followed by what Jan Peter Balkenende, the prime minister, called “a maelstrom of violence”.

There have been more than 20 arson attacks on mosques, churches and schools — the latest early yesterday on a mosque in the southeastern village of Helden. The spiral of attack and counterattack has shattered the image of Dutch society as one that cherishes consensus and abhors conflict.

Jozias van Aartsen, the parliamentary Speaker, warned yesterday: “Jihad has come to the Netherlands.”

Several politicians — including Ayaan Hirsi Ali, a liberal MP of Somali origin and van Gogh’s co-producer — were forced into hiding after death threats from Islamic extremists, and a poll revealed that 40% of the Dutch now hope their 900,000 Muslim neighbours no longer feel at home. Some 80% want tougher policies against immigrants.

Like its counterparts in Britain, the Dutch secret service recruited Muslims and Arabic speakers after the September 11, 2001, terrorist attacks on America. Among them was a man named only as Outmar Ben A, a Dutch-born Moroccan taken on last year. Intelligence officials believe he was the mole. He was arrested in September and has been charged with supplying documents showing what the security services knew about the cell.

The documents included a file on one of the cell’s alleged leaders, Samir Azzouz, an associate of Bouyeri, who has been charged with planning attacks on the Dutch parliament, Schipol airport and a nuclear reactor.

The material was unearthed during a search of Bouyeri’s home after the murder. Bouyeri, who has refused to co-operate with interrogators, featured in numerous intelligence reports that linked him with suspected terrorists. One of them was believed to have been involved in the Casablanca bombings that killed 43 people in May last year.

Police records have revealed growing concern over the young man’s desire to take part in jihad. “Bouyeri appears to be becoming increasingly radical, has changed his behaviour in a short time and now shouts out Koranic texts,” said one report.

In October last year, anti-terrorist police raided a number of buildings, including Bouyeri’s flat. But despite finding a “jihad martyr’s will” that suggested he was planning a suicide operation, they released him without charge.

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