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Perkhotin rushes over to the house of Mikhail Makarovich Makarov, the district commissioner of police, where a party is taking place. When he arrives, he discovers that not only are all the important officials there, but they have also already been notified of Fyodor Karamazov's murder by the servant Marfa Ignatievna.
What happened was this: Marfa is woken up by a terrible scream from Smerdyakov, the kind that always began his epileptic fits. She notices Grigory is missing, then hears groaning from the garden, where she discovers a bloodied Grigory much as Dmitri left him in. She rouses her neighbor, Maria Kondratievna, her daughter, and the visiting Foma, and they all go to Fyodor Karamazov's house, where they discover his dead body. Marfa then runs off to the deputy commissioner's.
Perkhotin, Makarov, and the other officials head over to Fyodor Karamazov's house to investigate. After surveying the scene, they dispatch the deputy commissioner off to Mokroye to keep watch on Dmitri while they get the paperwork settled for his arrest.
This is why, when Dmitri had previously met Trifon Borisivetch outside (Book 8, Chapter 8), the innkeeper looked concerned – he had already been notified by the deputy commissioner of Dmitri's impending arrest.