explosives used in the gun and suicide attacks in paris, his whereabouts are unknown. and prosecutors admitted they are not close to solving the puzzle. "we're working on an enormous amount of cases," said frederic van hoy. "they're becoming more and more worrying and violent." authorities can't explain how abdeslam was able to disappear for months only to be captured a few blocks from his home in a brussels neighborhood. interior minister jan jambon told us isis is growing more sophisticated. what took so long? >> they know everything about arms. they know everything about communication techniques. they hide. they have a network. it's professionals against professionals. >> reporter: but there is another reason abdeslam may have been able to hide in plain sight. according to anne speckard, an expert in violent extremeism, who has interviewed dozens of islamic radicals here. >> these are really brutal,