Go to the marriage certificate place in maryland where they married under Powell and Thomas, to make sure they were married or married in that state. or go to ancestry.com and make a general census search with no specific state and then click on all the similar names of your relatives, under powell or thomas. If they have a male son search under his name in the census, male children sometimes keep the name of their father,maybe you will find the family, or search the children under the mothers maiden name. Look up The surname Thomas in the war records, this will tell you where he died and was buried and the name of his wife. look up the female maiden name powell or the male death records.
If they were slaves, slaves were often not recorded on the census, then you would have to find the slave master and the will and sometimes on the wills they record the slaves name.
If they were immigrants then you will have to find the ships manifest and the port of entry.
If they were living as freedmen with the Indians then go to www.accessgenealogy.com for the Indian census and rolls.
Also, try church of ladder day saints genealogy.