adverb
Used when examining, explaining, or investigating something thoroughly, paying attention to fine points. Often appears in formal or written contexts.
事件の経緯を仔細に調べる必要がある。
We need to investigate the circumstances of the incident in detail.
彼は契約書を仔細に検討した。
He examined the contract minutely.
詳しく is a more common and neutral way to say 'in detail', while 仔細に is more formal and often implies a careful, minute examination.
From the noun 仔細 (しさい, 'details; circumstances') combined with the adverbial particle に. The kanji 仔 and 細 both relate to smallness or fineness, reflecting the sense of minute examination.