Complex sentence: The only person to notice Voldemort was Frank Bryce, the Riddles' gardener, who fell under suspicion from the Muggle police, as Bryce was the only person who had keys to the house and the door had not been forced.
Split into simpler sentences: The only person to notice Voldemort was Frank Bryce, the Riddle's gardener. The Muggle police suspected Frank, as he was the only one who had keys to the house and the door hadn't been forced.