Amsterdam, Netherlands ? A five-page letter pinned to the body of a Dutch filmmaker brutally murdered after making a movie critical of Islam called for Muslims to rise up against the “infidel enemies” in the West.

Other messages — later left at the sidewalk shrine where Theo van Gogh’s throat was slashed — dripped with equal venom against radical Islam. “Enemies live among us,” read one missive in a bed of flowers, votive candles and crosses.

Europe’s complex interplay with Islam appears to stand at a tipping point, and Tuesday’s slaying of the 47-year-old filmmaker as he was riding his bike down a busy Amsterdam boulevard could indicate one direction in which it is headed.

“The Muslims say they’re scared,” said mourner Nicolette Toering. “No, we’re scared.”

Dutch authorities were investigating whether the chief suspect in the slaying, a 26-year-old Dutch-Moroccan man arrested shortly after the attack on terrorism-related charges, acted alone out of rage or had links to wider extremist networks. Police have detained several other suspects facing charges including conspiracy to murder.

The letter stuck to the victim’s body threatened death to Somali-born lawmaker Ayaan Hirsi Ali, who scripted Van Gogh’s last film, “Submission,” which criticized the treatment of women under Islam.

It also predicted the downfall of the “infidel enemies of Islam” in Europe, America and the Netherlands.

Heightening fears nationwide, two Dutch men were arrested late Friday for allegedly posting a video on the Internet calling for the beheading of right-wing lawmaker Geert Wilders for perceived insults to Islam.

Prosecutors said the video was posted last month and offered “paradise” for whoever beheads Wilders. The lawmaker, who said after Van Gogh’s slaying that he would form his own anti-immigration party, has been threatened before and remains under police protection.

The attack and threats have underscored the hard political and social choices that European leaders face about Muslims and the wider Islamic world.

In December, European Union leaders will decide whether to overlook widespread public objections and move ahead with membership talks with Turkey, a Muslim nation of about 70 million people and a galloping birthrate that could push it past Germany’s population in a generation.

European police agencies have sharply boosted cooperation against suspected Islamic terrorist groups after the March train bombings in Spain that killed 191 people. Washington’s European allies in Iraq are reassessing their levels of military and commercial support after waves of attacks, kidnappings and beheadings blamed on Islamic militants.

EU officials last month signed the text of a proposed EU Constitution that still could face opposition from voters demanding a clear reference to Europe’s Christian history.

But those big issues fade on the streets of many European centers. Here — even in places like tolerant Amsterdam — it’s often expressed as a gnawing feeling that militant factions in Islamic immigrant communities are gaining ground and chipping away at values such as free speech and secular politics.

“There is a general feeling that a social collision is becoming inevitable,” said Jan Rath, co-director of the Institute for Migration and Ethnic Studies at the University of Amsterdam. “People think it’s been building for years and now finally coming to the surface.”