Here is what Baza writes, known for leaks from the security forces - teenagers who were arrested during the attack over Kitai-Gorod are put on preventive registration in the PDN. Among the participants in the inspection were apparently those who had already come to the attention of the police in previous attacks. After that, preventive conversations were held with some minors and they were recorded.
When the security forces asked why they came to Kitai-Gorod again, the youths answered that they had a meeting. Some also searched their personal belongings - they looked for drugs, knives, and personal protective equipment (whatever that matters in context). In this video the guy talks about the kidney.