MONTEZUMA, Iowa (AP) — Part of the case against a former farmhand charged with first-degree murder in the killing of 20-year-old University of Iowa student Mollie Tibbetts appears to be in jeopardy because of a botched Miranda warning.

The Des Moines Register reports that prosecutors have agreed to exclude some statements that 25-year-old Cristhian Bahena Rivera reportedly made to police. Court documents filed Friday by prosecutors acknowledge that the initial Miranda warning given to Rivera around 11:30 p.m. on Aug. 20, 2018, failed to inform him that anything he said could be used against him in court.

His Miranda rights were accurately read to him a second time at 5:50 a.m. Aug. 21, as police were in a cornfield where Tibbetts’ body was found.

Rivera is accused of killing Tibbetts, who disappeared while out for a run near Brooklyn, Iowa. Investigators found her body a month later. Officials say she was stabbed to death.