Muhammad said he had found a better life in Russia. After emigrating from Tajikistan last fall, he began driving delivery vans in Siberia, enrolled his children in a local school, applied for a Russian passport and started planning to buy an apartment with the savings from his much higher salary.

The arrest of a group of Tajik citizens accused of carrying out the attack that killed 145 people at a Moscow concert hall last month has upended those plans, filling Muhammad with fear of being swept up in the ensuing crackdown on the Central Asian migrants who prop up Russia¡¯s economy.

The attack, he said, has erased all the efforts his family made to fit into society. In a phone interview from the city of Novosibirsk, he added that he would move back to Tajikistan if the police or nationalist radicals were to target him.