Father jailed for 21 years for murdering schoolgirl has sentence overturned after judge finds cops covered evidence.

A father who spent 21 years on death row for murdering a schoolgirl while trying to steal her earrings could soon be free after a federal judge overturned his conviction, claiming police 'covered up evidence.'

Senior U.S. District Judge Anita Brody called the case against James Dennis 'a grave miscarriage of justice' and said 'improper police work' led to the now-42-year-old being unfairly locked up for two decades.

In a ruling on Wednesday, she ordered Dennis released from prison unless prosecutors retry him within six months.

Brody said Dennis was convicted based on 'shaky eyewitness identifications' following the shooting of Chedell Williams, 17, at a transit stop in Philadelphia's Fern Rock section in October 1991. 

But other witnesses described Williams' killer as much taller than Dennis, who is 5-foot-5, and weighed more, according to the ruling.

Williams was at the train station with a female friend. Witnesses told police that at least two men were involved in the shooting, and possibly a third as a getaway driver, but Dennis was the only person charged. The men demanded her $450 earrings, and she handed them over, but they shot her anyway.

The weapon used in the shooting and the earrings were never recovered and police found no forensic evidence against Dennis.

Williams' friend told relatives that she knew the two assailants from high school and knew their nicknames, however she had never seen the men shown to her in the line up. The relatives passed the information to police, but they never followed up on it, Brody found.

'Improper police work characterized nearly the entirety of the investigation,' the judge said, according to NBC NEWS

Dennis, who was 20 and had only a minor drug arrest on his record when he was charged in Williams' death, maintains that he was on a bus at the time of the shooting and offered up an acquaintance to corroborate his story but he was convicted in 1992.

'The Commonwealth of Pennsylvania covered up evidence that pointed away from Dennis,' Brody added. '(It) has committed a grave miscarriage of justice in convicting Dennis and sentencing him to die for this crime.'

The judge also questioned the identification techniques used during a police photo array and police lineup, which have come under fire in numerous cases from the time.

He was already a father and had a child born after he went to prison.

'For him, as a father, for him to have been accused of this horrible crime has really taken a toll on him,' pro bono lawyer Ryan Guilds, who has worked on his appeal since 1999, told NBC.   

However, Philadelphia District Attorney Seth Williams said he was disgusted by the judge's decision and his office was reviewing their options.

'The defendant was convicted for the brutal murder of a teenage girl on her way home from school, in broad daylight,' Williams told NBC. 'Three passersby saw him do it and identified him in court, without any possible motive to lie.'

Dennis, who is now a grandfather, hopes to be reunited with his family as soon as possible.

So many people have wasted their lives in jail for no fault of theirs. Unbelievable.