Abdullah, tall and muscular, with a shaved head and closely cropped goatee, sat on a metal bunk in his cell and described how he got religion.
"When I was in La Sante, I read books about the Prophet," he said, referring to a notorious Parisian detention center, the third of five jails where he has spent time during the past two years for dealing drugs and stealing cars.
When he arrived at the fourth, Fleury-Merogis, Europe's largest, another inmate gave him a DVD about the life of Muhammad, and later, while enduring a three-week stint in solitary confinement, he vowed to devote himself to Islam.
"People here find God," he said.
In less than a decade, there has been a radical shift in France's prison population, a shift that officials and experts say poses a monumental challenge. Despite making up only 10 percent of the population, Muslims account for most of the country's inmates and a growing percentage of the prison populations in many other European countries, an indication of their place at the bottom of the continent's hierarchy.
With radical strains of Islam percolating through Europe, authorities are unsure how to address the spiritual needs of the prisoners while guarding against the potentially toxic mix of extremist ideology and a criminal past.
France's prison population has risen by 20 percent in the past three years, largely because of aggressive pursuit of lower-level crimes. The proportion of Muslims in prison has been growing even faster, reflecting the relative youth of Europe's largely Muslim immigrants.
While there are no official data on issues of race and ethnicity in much of Europe -- it is in fact illegal in many places -- experts on prison populations agree on the new disproportion of Muslims here and elsewhere.
Two months ago Pierre Raffin, the director of La Sante detention center, warned officials looking into the role of religion in France that extremist proselytizing in prisons was growing.
Other countries face the same problem. Most famously, Richard Reid, who tried to blow up a Miami-bound airliner in December 2001 using a bomb in his shoe, converted to radical Islam while in a British jail.
Those who are detained or convicted of terrorist-related crimes are not always separated from the larger prison population and are often ready to act as spiritual guides at a time when there are few mainstream Muslim chaplains.
Farhad Khosrokhavar, an Iranian-French scholar who has written on Islam in prisons, says an Islamic underclass is developing across Europe and, at its margins, is increasingly sympathetic to the coalescing ideologies of political Islam.
"Islam is becoming in Europe, especially France, the religion of the repressed, what Marxism was in Europe at one time," he said.