I lean toward Paul. But simply put, we don't know for sure. In some places it reads like Paul. In other places it doesn't. At least the Paul we are familiar with in his other epistles. Of course it has been mentioned that Hebrews is a bit "academic" compared to some of the other letters so that could account for the difference in tone.

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Donald Jacobs
Marysville. WA
Cascade Church (CRCNA)

Cum vero infirmor tunc potens sum.