pre-noun adjectival
the ... in question; the very ...; the ... itself
Pre-noun adjectival (rentaishi) that specifies the exact person or thing being discussed, often with a nuance of 'the very one' or 'the one at issue'. Common in news and formal contexts.
当の本人は何も知らなかった。
The person in question knew nothing about it.
The very person in question knew nothing at all about the rumor.
当の事件について詳しく説明してください。
Please explain the incident in question in detail.
当の商品はもう売り切れていた。
The very product in question was already sold out.
当該 is a stiff, formal/legal term meaning 'the relevant ...', whereas 当の is more neutral and usable in everyday writing and speech.
当の is an adnominal form placed before a noun to mean 'the very ... in question', while 当 attaches directly as a prefix (as in 当社, 当日).
the person in question; the very person
Compound of 当 (tō, 'this' or 'the relevant') + の (no, attributive particle). The exact historical derivation is uncertain, but it functions as a fixed rentaishi in modern Japanese.