A former Missouri substitute teacher who was accused of raping a student and threatening her to remain silent has been found dead in his jail cell. 

Brandon Holbrook, 30, was found dead in a St. Louis County Justice Center cell after being charged with three counts of statutory rape and six counts of statutory sodomy, St. Louis Post-Dispatch reported.

Holbrook was alone in his cell when he was discovered unresponsive on Monday afternoon. A cause of death is not yet known but a county executive told the outlet that there were no indications of drug use or suicide.

Holbrook had been charged earlier in the month with raping one of his 14-year-old female students while she was attending Bernard Middle School. 

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Brandon Holbrook, Missouri substitute teacher

Missouri substitute teacher Brandon Holbrook. (St. Louis County Police Department)

Police say Holbrook groomed the girl on social media after meeting her at the school in May. Holbrook was accused of raping the girl at her home on three different occasions while making veiled threats against her if she told law enforcement or her family.

Police are investigating claims from two other alleged victims of Holbrook.

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Outside of St. Louis County Jail

Two women enter the Buzz Westfall Justice Center in Clayton, Missouri. (JEWEL SAMAD/AFP via Getty Images)

Holbrook is the third inmate to die at the St. Louis County Jail this year after five inmates died at the jail in 2019.

Hollbrook's attorney Scott Rosenblum told St. Louis Post-Dispatch that his client's death seems "very unusual" and that he knows for a "fact" that his client was not suicidal.

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The jail did not respond to a request for comment from Fox News Digital.