Gerald Ardell Hale, 81, was arrested for sexually abusing a juvenile family member. The assault happened in 2008, Farmington police Det. Heather Chavez said.
Hale has a criminal history of child abuse that dates back to 1969, but investigators believe he got away with more sex assaults against children than he was charged with.
Hale allegedly told investigators this week he started sexually abusing children when he was 10 years old, Chavez said.
"He lost count of the victims," Farmington police Cpl. Clay Raybon said. "He never really stopped. It was a perpetual thing. He told us he lost track of how many victims he had."
Chavez said Hale described at least 25 assaults during his interview with police.
"I honestly don't know how many victims there were," Chavez said. "He made numerous disclosures prior to incarceration."
Hale previously worked as a substitute teacher for the Farmington Municipal School District, prior to pleading guilty to four counts of criminal sexual contact of a minor in 1995, according to a state courts website.
The exact dates of his employment with the school district were not available Thursday evening, Superintendent Janel Ryan said.
Hale was arrested for child molestation in California in 1969 and 1970, Chavez said. He was arrested in Farmington in 1995 and again in 2005 for raping a child, though it did not appear he was convicted of the most recent charge, according to a state court website.
Hale was a registered sex offender at the time of his arrest on Thursday.
Farmington police detectives started investigating Hale last month when Hale's family member came forward and reported sexual abuse that took place in 2008, Chavez said.
The victim waited several years to report the assault because the person was afraid of Hale, Chavez said.
The statute of limitations has expired on all of the assaults Hale allegedly admitted to except for the 2008 molestation. He was charged with one count of criminal sexual contact of a minor and is being held at San Juan County Adult Detention Center on a $170,000 bond.