An ultra-radical Islamic ideology mixing zealot-like devotion and holy war creed is drawing more scrutiny in anti-terrorist probes from the Middle East to Europe — with increasing indications that its base on the fringes of Islamic extremism could be widening. In existence since the 1960s, al-Takfir wa al-Hijra has offered intellectual inspiration to al-Qaida and other militant groups. But authorities now worry about followers becoming more aggressive with recruitment and retaliation against perceived foes of Islam, such as Dutch filmmaker Theo van Gogh. Officials in the Netherlands say the Dutch-Moroccan suspect — accused of killing Van Gogh on a busy...