They had after painstaking investigation, found out who was one of the English-speaking scientists working at the facility, Yuri Romanov, and used the Imperius curse on him, giving him instructions to unlock the door to Building C at exactly 3:00pm on this day. They wanted to meddle with his mind as little as possible; meddling led to possible brain damage especially with Muggles, so they hadn't ripped the passcodes from his mind. Besides, they needed his eye and thumbprint too, to open the doors – and if they used polyjuice and replaced him, there was still the problem that none of them spoke Russian.