A 15-year veteran of San Francisco's public school system is facing federal felony charges, after he was allegedly found in possession of at least one image of minors engaged in sexually explicit conduct.

Christian Lancaster, who the Ex says taught at Thurgood Marshall High School in the Bayview "from at least 2002" until January 8, 2016, "was charged in a July 14 federal grand jury indictment with one count of knowingly possessing and gaining access to one or more images of minors engaging in sexually explicit conduct," Bay City News reports.

According to the indictment, which wasn't unsealed until his initial court appearance Thursday, Lancaster "had or accessed at least one image of prepubescent minors engaging in sexually explicit conduct for an unknown amount of time until last September," the Ex reports.

San Francisco Unified School District spokesperson Gentle Blythe tells the Chron that Lancaster had “no record of wrongdoing” during his lengthy career at the school, and "had passed background checks prior to being hired."

According to Lancaster's bio on the Thurgood Marshall site (which has since been removed), the former Government and Economics teacher was born in Stockton and had lived in San Francisco for nearly 20 years.

“His fascination with history and government is equaled only by his commitment to challenging students to learn about the world around them,” his bio read.

At his appearance before U.S. Magistrate Donna Ryu in Oakland Thursday, Lancaster surrendered and was taken into custody. He'll remain there until at least July 27, the date of his bail hearing.

U.S. Attorney Brian Stretch says that Lancaster's defense attorney has yet to be named. If convicted, Stretch says, Lancaster could face a fine as high as $250,000 and up to 20 years in prison.