BALTIMORE — The city’s top prosecutor acted swiftly in charging six officers in the death of Freddie Gray, who suffered a grave spinal injury as he was arrested and put into a police van, handcuffed and without a seat belt.

But getting a jury to convict police officers of murder and manslaughter will be far harder than obtaining arrest warrants.

Legal experts say the case is fraught with challenges. The widely shown video that captured the nation’s attention shows Gray, 25, being loaded into a police transport van, but not what happened once he was inside. Other than the accused officers, the only known witness is a convicted criminal later placed in the van’s other holding cell, unable to see what was happening with Gray.

By bringing charges less than two weeks after Gray’s death, State’s Attorney Marilyn Mosby said “no one is above the law.”

But within hours, the city’s police union questioned the prosecutor’s impartiality, accusing her of a rush to judgment and demanding she recuse herself from the case. Even some of those who support Mosby’s stand worry further violence might erupt if she fails to win convictions.

Alan Dershowitz, professor emeritus at Harvard Law School, suggested that Mosley’s actions were motivated by political expediency and short-term public safety. He called the charges “outrageous and irresponsible,” especially the second-degree murder count filed against the van’s driver under a legal principle known as “depraved heart.”

To win a conviction, prosecutors will have to convince a jury that van driver Caesar Goodson acted so recklessly that he knew his actions could take Gray’s life.

But it is rare for law enforcement officers to be charged following fatal encounters with suspects, much less convicted by jurors often predisposed to give extra credibility to the accounts provided by police.

And the video evidence is murky, with no visual evidence the officers purposely beat Gray.