He thought the police might try to dissuade him from his plan, or even prevent him. The prospect of a confrontation made him nervous. Peter knew he had to make a decision quickly.
Logically, the safest option was to open the door, talk to the police, and abandon his plan. But the thought of giving up on his investigation was difficult. The iceberg was more than a curiosity to him now; it was a mystery he felt compelled to solve.