The evidence linking the suspect to the crime was _____ and lacked substantial proof.

tenuous 意為「薄弱的、脆弱的、勉強的」，常用於形容證據、論點、關係站不住腳。與 lacked substantial proof 完美呼應，整句譯為「將嫌犯與犯罪連結的證據薄弱且缺乏實質證明」。