(CNN) A former Oklahoma City police officer who was convicted of raping multiple women has been denied an appeal.

Daniel Holtzclaw's attorneys raised the appeal by proposing seven errors in the conviction, according to documents posted on the court's website. The judge denied all seven Thursday.

He was sentenced in 2016 to 263 years in prison on charges including rape after preying on African-American women over a period of six months.

Prosecutors said Holtzclaw targeted victims in one of Oklahoma City's poorest neighborhoods. They said he chose them based on their criminal histories, assuming their drug or prostitution records would undermine any claims they might make against him.

He would then subject them to assaults that escalated from groping to oral sodomy and rape, according to the testimony of 13 victims.

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