Mastering Bitcoin – Who is the originator of the secp256k1 standard?

There are various accounts about who created the secp256k1 standard. For example:

Mastering Bitcoin, p.61:

Bitcoin uses a specific set of elliptic curves and mathematical constants defined in a standard called secp256k1 established by the National Institute of Standards and Technology (NIST).

theymos in this thread on r/crypto:

secp256k1 is not a NIST curve. It was created purely by SECG. SECG also publishes some NIST curves in their standards (e.g. secp256r1, aka P-256), but secp256k1 is not one of them.

So who made it? Is there a ‘regular’ publication somewhere?