Michael Palmatier, 48, was taken into custody after being arrested on suspicion of sexual conduct with a minor, kidnapping and indecent exposure in Arizona, US.
The fireman told a friend who later confronted him he “fucked up” while “extremely intoxicated” at a 50th birthday party on April 21.
He is alleged to have performed a sex act on the girl, who was asleep in a bedroom during the party.
Palmatier claimed he thought he was kissing an adult’s mouth rather than the girl’s genitals, court documents say.
Maricopa County Sheriff’s Office said her family was first alerted to the assault by the girl’s aunt who saw the kid leave her room in tears.
Witnesses noticed Palmatier “abruptly leave” the party and a male friend of the alleged victim’s family went to his house to confront him, according to a police report.
The records add: “Michael answered the door and appeared to be extremely intoxicated.”
He then repeatedly said: “I f***ed up.”
“Yeah I have a sick side to me, like a sexually sick side to me, but not in that way”
One of Palmatier’s co-workers told investigators he had a drug and alcohol problem.
Palmatier claims he took medication earlier on the day of the party as he had injured his neck while having sex with his girlfriend.
The fireman “started to cry and apologise” when a co-worker talked to him about the age of his victim and said he “let his demons get out”.
Deputy sheriffs recorded a phone call where the accused was grilled about how he could confuse a child with an adult.
He claims he went onto the rooms because he “sexually desired” one of the adults.
The sheriff’s records said: “Michael admitted the entire thing was wrong.”
Palmatier also said: “Yeah I have a sick side to me, like a sexually sick side to me, but not in that way.
“Eight-year-olds don’t make that s**t up.”
Palmatier has been released from custody and ordered by a judge to wear an electronic monitoring device.
He resigned from the Gilbert Fire Department after his arrest last Wednesday and will appear in court on May 1.