Matthew 27:51-53 BDC: Who were the “they having come out of the tombs after his [Jesus] resurrection [that] went into the holy city and appeared to many”?

These verses are one of the bigger Bible difficulties talked about recently, but fail to be in some of the more popular books on Bible Difficulties, such as Norma L. Geisler’s Big Book of Bible Difficulties and Gleason L. Archer’s Encyclopedia of Bible Difficulties. What really happened?