(Reuters) - The Baltimore police investigation into the death of a black man from injuries sustained in police custody fails to support some charges filed by the city prosecutor, CNN reported on Thursday.

Citing officials briefed on the separate probes by prosecutor Marilyn Mosby and police into the death of Freddie Gray, the television news network said the lack of support for charges from the police findings could allow lawyers representing the officers to undercut the prosecution.

Gray, 25, sustained spinal injuries after being arrested, and his death on April 19 sparked protests and a day of arson and looting in the largely African American city.

Mosby charged six officers on Friday with counts ranging from false imprisonment to manslaughter and second-degree murder. She made her announcement hours after the state medical examiner ruled the death a homicide and a day after police turned over their findings about Gray's arrest.

Officials familiar with the probes say the homicide investigation run by police at most contemplated a manslaughter charge, not second-degree murder as Mosby charged one of the officers, Caesar Goodson.

To win conviction for murder, prosecutors must prove intent to kill. Manslaughter relates to unintentional killings.

Homicide investigators briefed by the medical examiner's office believed the autopsy report would likely find the cause of death to fall short of homicide, one official familiar with the case told CNN.

Mosby said that the medical examiner concluded that Gray's fatal injury occurred in a police transport van taking him to a station house.

Lawyers for two officers have challenged a key part of Mosby's case, that a knife found on Gray was legal in Maryland and thus officers did not have a right to arrest him. The police investigation found that the knife was illegal under the city code.

A police spokesman did not respond to a request for comment. Mosby said in a statement that she refused to litigate the Gray case in the media.

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"These unethical disclosures are only damaging our ability to conduct a fair and impartial process for all parties involved," she said.

Police Commissioner Anthony Batts told CNN on Tuesday that Mosby called him about 10 minutes before she disclosed the charges.

Baltimore Mayor Stephanie Rawlings-Blake asked the U.S. Justice Department on Wednesday to investigate the police department for civil rights violations.

(Reporting by Ian Simpson in Washington)