You don't really need the past perfect form - had had - there, though it wouldn't be wrong. The past simple "had" is OK, because in situations like those, the 'past simple' is usually the more natural choice, since the order of the events is made clear by the words such as "after" or as in your "before".
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vkr6078"The building was evacuated after they found/had found one suspected thing"The underlined part is not natural English. Native speakers say "suspicious item" https://www.ucalgary.ca/emergencyplan/emergency-ins