Revelation 6:9: “...I” [John] “saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:”
Revelation 6:11: “And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren," [The rest of the 144,000] "that should be killed as they were, should be fulfilled.”