MONTEZUMA — 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 an officer failed to properly read him his Miranda rights.

Prosecutors have agreed to exclude at trial some statements that 25-year-old Cristhian Bahena Rivera made to police, the Des Moines Register reported . Court documents prosecutors filed Friday acknowledge that the initial Miranda warning given to Rivera around 11:30 p.m. on Aug. 20, 2018, was incomplete because the officer “inadvertently” failed to inform Rivera 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. the following day as police were in a cornfield where Tibbetts’ body was found.

Tibbetts disappeared while out for a run July 18, 2018, in Brooklyn, Iowa. Investigators recovered her body a month later. She had been stabbed to death.

Prosecutors agree in their filings Friday that any statements Rivera made between 11:30 p.m. and 5:50 a.m. should be suppressed but argue they should be able to use those statements to rebut testimony. Defense attorneys for Rivera want the statements fully suppressed.

Chief Deputy Attorney General Eric Tabor on Friday declined to comment or answer questions about which of Rivera’s statements would be excluded from trial evidence.

Police have said Rivera led them to Tibbetts’ body after confessing during a lengthy interrogation to following Tibbetts in his car, getting out on foot and chasing after her. Rivera told investigators that he panicked after Tibbetts threatened to call police on her cellphone. Police said Rivera told investigators he blacked out and later came to when he was unloading her bloody body from the trunk of his car.

Rivera is set to appear in court Oct. 22, when a judge will decide what evidence will be allowed at his trial, which is set to start in February.