JOEL - Events leading to the separation of the ten tribes of Israel are linked to the division of the Israelite monarchy at about 930 B.C. After Shalmanezer overran the kingdom of Israel and held them captive in Assyria, the ten tribes were led to the north where they remain lost to this day. Jared led his people from "the great tower" in Mesopotamia(Gen. 11:1-9) to the Western Hemisphere much earlier than this, at around 2200 - 3000 B.C. and were therefore not involved in the division of the tribes. Even though they left their homeland, they are not considered members of any of the lost ten tribes. If they were truely "lost" we would not be reading about them in the Book of Mormon would we? :-)
A scripture in the Book of Mormon gives us another clue that they were not part of the lost ten tribes:
"And it shall come to pass that the Jews shall have the words of the Nephites, and the Nephites shall have the words of the Jews; and the Nephites and the Jews shall have the words of the lost tribes of Israel; and the lost tribes of Israel shall have the words of the Nephites and the Jews." (2 Nephi 29:13)
In this revelation given to the prophet Nephi, God is talking about a distant future event when "the Nephites and the Jews shall have the words of the lost tribes". His wording makes it clear that He is talking about three distinctly separate groups of people with their own separate records. A few hundred years later after this statement the Nephites received the records of the Jaredites(Mosiah 28:11), which were added to the Book of Mormon by Moroni(Ether 1:1-2), thus becoming a part of the Nephite records.
Therefore, we have the records of the Jews(Bible) and the records of the Nephites(Book of Mormon), which includes the Jaredite records; but are still waiting for that time when the records of the lost ten tribes are found to join them. The lost tribes are still truely lost and as far as we know, no records of them have been found yet.