Authorities in St. Petersburg, the hometown of President Vladimir Putin, deported migrants en masse after the deadly attack on the Crocus City Mall concert hall in Moscow, according to human rights organization Pervyi Otdel, independent Russian media reported.

More than 64 foreigners were expelled from the Vyborz district of St. Petersburg alone on Thursday as part of what Russian authorities are calling “Operation against migrants,” a lawyer for an NGO said.

“Detention facilities for foreign nationals are overcrowded, surrounded by special vehicles and buses heading to the airport,” the lawyer told The Moscow Times.

“Raids are also conducted in dormitories and apartments. All detention centers are overcrowded,” he said, as quoted by Meduza.

According to him, migrants are being deported en masse from Russia. Similar raids were reported in Moscow and other Russian cities.

Several buses with migrants were also heading to Pulkovo International Airport on Friday afternoon, he added.

The countries where the migrants were sent are not specified, although it is known that those who come to work in Russia mainly come from poor countries in Central Asia.

According to Russian authorities, four citizens of Tajikistan are accused of carrying out the deadly attack on the Crocus City Mall concert hall on March 22, which killed 144 people and injured 382 others.

Four other suspects who were remanded in custody this week are also from Tajikistan. The ninth alleged accomplice is from Kyrgyzstan.

The Islamic State group ISIS-K claimed responsibility for the massacre. Russia has blamed radical Islamists, Ukraine and its Western allies for Russia’s deadliest attack since the Beslan school siege in 2004.