Nine Liars by Maureen Johnson

While on a school trip to London, a teen detective is drawn to a notorious 1990s cold case involving a student theater troupe whose drunken night at a country estate ended with two friends murdered in a locked woodshed. Interviewing the now-grown survivors, she uncovers layers of jealousy, ambition, and long-nurtured lies that could explain what really happened, even as complications in her own friendships and romance threaten to cloud her judgment. As the secrets close in, the investigation escalates into a dangerous confrontation that forces her to decide whom to trust and how far she’ll go for the truth.

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