The mother of one of the suspects in the bloody attack on a concert hall near Moscow last month wept as she talked about her son.

How, she wondered, did he go from the bumpy, dirt roads of their village in Tajikistan, in Central Asia, to sitting, bruised and battered, in a Russian courtroom accused of terrorism? Even though he spent five years in Tajik prisons as a teenager, she said he never exhibited signs of violent extremism.

“We need to understand — who is recruiting young Tajiks, why do they want to highlight us as a nation of terrorists?” said the mother, Muyassar Zargarova.

Many governments and terrorism experts are asking the same question.

Tajik adherents of the Islamic State — especially within its affiliate in Afghanistan known as the Islamic State Khorasan Province (I.S.K.P.), or ISIS-K — have taken increasingly high-profile roles in a string of recent terrorist attacks. Over the last year alone, Tajiks have been involved in assaults in Russia, Iran and Turkey, as well as foiled plots in Europe. ISIS-K is believed to have several thousand soldiers, with Tajiks constituting more than half, experts said.

“They have become key to I.S.K.P.’s externally focused campaign as it seeks to gain attention and more recruits,” said Edward Lemon, an international relations professor at Texas A&M University who specializes in Russia, Tajikistan and terrorism.