Nine people were arrested Friday by Tajikistan’s state security service in connection to the terror attack at a Moscow concert hall that left 143 people dead and scores more injured earlier this month, Reuters reported, citing a Tajik source. Russian state media also confirmed nine were arrested in Tajikistan, but the identities of those arrested were not released. The arrests come amid an international probe into the attack, which has so far resulted in the arrests of 16 people. Tajikistan, a former member of the Soviet Union that hosts a Russian military base, has also reportedly rounded up the families of the suspected gunmen for questioning by Russian investigators, sources told Reuters. A motive behind the attack remains murky, but the Islamic State immediately claimed responsibility. U.S. officials said they have intelligence suggesting the attack was carried out by the network’s Afghan branch, Islamic State Khorasan. Despite that intelligence, Russia has claimed the gunmen were linked to “Ukrainian nationalists,” suggesting Kyiv could have been behind the attack. Ukraine has denied any involvement, however, and the U.S. has dismissed those claims as being baseless propaganda.