The incarceration rate of Muslims in the U.K. is exploding, according to a report published Friday – but officials have no idea why.

The U.K. has seen a 50 percent surge in the number of Muslims incarcerated within their prisons over the last ten years, however, experts said that “radicalization” is not the cause, Yahoo News reported.

Despite only making up five percent of the British population, Muslims comprise 15 percent of the British prison population.

However, a lack of data means officials are not sure why Muslims are flooding prisons at such an alarming rate, Labour MP David Lammy said.

“The lack of transparency undermines accountability,” Lammy said.

The report, commissioned by David Cameron in 2016, states that officials have no idea what is driving the trend.

“We know far too little about what has been driving this trend,” the report concludes. “Are large proportions of prisoners converting to Islam once they are in custody? We simply do not know. This gap needs to be taken seriously.”

Europe’s top anti-terrorism expert said in late August that the U.K. is home to an estimated 35,000 Islamic extremists – more than any other country in Europe.

Out of that number, 500 are considered to be so dangerous that they require “special and constant attention.”

European jihadists, especially ones from the U.K., are said to be the cruelest of all jihadists, a former female ISIS member said.

The woman, known only as Hajer, fought in an all-female ISIS brigade known as Al-Khansaa.

“You know, Europeans are more sadistic and violent than the others,” Hajer said. “For example, the British female fighters boast of brutal torture. The one who used the ‘biting’ tool is British – pure British.”

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