I sent an e-mail to Father Carlos Martins, the primary contact of Treasures of the Church (a relic exhibition that travels across North America). Part of their relic collection includes a hair of St. Maximilian Kolbe, who was martyred in a Nazi death camp and had his body cremated.
When I asked how the relic was obtained, given that his body was burned, I was told this:
The hair came from the period when he was prior of his monastery. The barber saved the trimmings.
So this relic was obtained was well before St. Maximilian Kolbe was transferred to Auschwitz and martyred. I asked for clarification of whether a first-class relic needed to be collected before death and Fr. Martins responded with thsi:
I do not know where you are getting your information, but it is incorrect.
A first class relic is any part of the body.
So according to Fr. Carlos Martins of the Catholic church, a relic is a first-class relic if it comes from the body, regardless of whether it is collected before or after their death. The hair of St. Maximilian Kolbe, which was collected years before he was martyred, is an example of a first-class relic.