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.