They were having slow progress in the investigation due to the fact that the killer was a shape-shifter. They hadn't even figured out yet that the killer was a shape-shifter. When they do, what they will find will be a booby trap prepared for them.
Kelvin had learned a lot in the two days he was masquerading as a worker in the investigation of his own crime. The fact that the Count had been prepared for him also made him decide to make some more preparations.
He had called off the attack that night, which gave him time to map out the skyscraper and prepare a suitcase in the staircase.
He also had enough time to acquire a new safe house and turn it into a silent death trap made with odorless poisonous gas and explosive gas.
He changed his tracks and identity several times. He even destroyed his hoverbike so that it won't be used to find him. But he didn't account for Thorin.