He enlisted in Company C of Kearney's 1st Dragoons on August 6, 1844, and was described as, "27 years old, 5 feet 9 inches tall, with brown hair, dark eyes and fair complexion.
During the siege of Taos Pueblo, "Burgwin's dismounted dragoons were ordered to charge through galling fire from the loopholes and secure lodgment against the west wall of the church.
The American soldiers were given free access to the prisoner and on February 8, Fitzgerald entered the jail, pulled out a pistol and shot Romero, killing him instantly.
He returned to his unit where he received some supplies and from there he headed north from Santa Fe, eventually reaching Colorado where he met up with Ceran St. Vrain and Lewis Garrard, the latter of whom wrote one of the surviving accounts about Fitzgerald.
Fitzgerald explained to Gerrard after the killing of Romero that he did it because "his brother having been murdered by Salazar while a prisoner in the Texas expedition against Santa Fe, he swore vengeance and entered the service with the hope of accomplishing it.