TOKYO — A police search for a missing Tokyo woman who had sought a suicide partner led them to a townhouse with nine mutilated bodies and coolers containing heads covered in cat litter, Japanese media reported Tuesday.

The townhouse tenant, a 27-year-old man identified as Takahiro Shiraishi, was arrested on suspicion of murder in an investigation that escalated into fears that he was a serial killer.

Accounts by the national broadcaster NHK and leading newspapers described Mr. Shiraishi’s second-floor townhouse, in Zama, Kanagawa Prefecture, southwest of Tokyo, as something out of a Halloween horror movie.

They quoted police investigators as saying they first found the heads of two victims in a cooler box — obscured by cat litter to mask the smell — at the entrance of the second-floor apartment, and other heads in cooler boxes elsewhere inside. Neighbors were quoted as saying a smell that resembled sewage had been wafting from the residence.