The Monitor of McAllen reports that a man from the town of Pharr was arrested Friday and is in the Hidalgo County jail.
A criminal complaint says the girl turned the letter in at Cesar Chavez Elementary School. Authorities interviewed the girl after a school counselor reported the letter.
The complaint says investigators believe the molestation occurred over a period of four years.
The man is charged with continuous sexual abuse of a young child and could face as many as 99 years in prison if convicted.