The agency said Wednesday that the additional victim was a girl who died in Oklahoma City. It didn't provide her name.
Storms that swept through the Oklahoma City area on Friday generated an EF5 tornado and violent flash floods. The tornado was the widest one ever recorded - 2.6 miles long.
Oklahoma emergency officials have said many of those who died in Oklahoma City were drowning victims who were swept away while taking shelter from the tornado in storm drainage ditches.
Nine of the 20 victims died from blunt force injuries typical from flying debris in tornadoes. The cause of death for two of the victims is listed as drowning.