Complex sentence: Grimson was considered as a suspect (although there are others) as he was friendly with the building manager (who had seen Grimson in the building on several occasions) and whilst Grimson was a suspect, investigators have noted that his other victims were men.
Split into simpler sentences: Grimson was considered as a suspect (although there are others) as he was friendly with the building manager. The building manager admitted that he had seen Grimson in the building on several occasions, and whilst Grimson was a suspect, investigators have noted that his other victims were men.