A judge declared a mistrial Friday in the case of a former Harvard graduate student accused of stabbing a teenager to death during a fight.

After 10 days of deliberations, the jury was unable to reach a verdict on the manslaughter charges against Alexander Pring-Wilson, 29, who was being tried for a second time.

Pring-Wilson said he acted in self-defense after he was attacked by 18-year-old Michael Colono and his cousin, Samuel Rodriguez, outside a Cambridge pizza parlor as he walked home from a bar on April 12, 2003.

Rodriguez testified that Pring-Wilson became enraged when Colono ridiculed him for stumbling home drunk.

The case attracted widespread media attention because of long-standing tensions between Ivy Leaguers and working-class Cambridge residents.

Pring-Wilson, the son of Colorado lawyers, was studying for his master's degree in Russian and Eurasian studies at Harvard. Colono, a high school dropout, had fathered a child at 15. He had earned his high-school equivalency diploma and was working as a cook at a Boston hotel when he was killed.

Pring-Wilson will remain free on bail. Prosecutors said they will put him on trial a third time.

"We will honor the memory of Michael Colono by continuing to fight for justice on behalf of him, his family, and the commonwealth," Middlesex District Attorney Gerry Leone said in a statement.

Pring-Wilson was convicted of manslaughter in 2004, but won a new trial eight months later when the state's highest court ruled in another case that juries should be allowed to consider a victim's violent history if it is relevant to a claim of self-defense.

During the second trial, jurors were given details about Colono's criminal record, including a 2001 episode in which he threw money in the face of a cashier at a pizza restaurant, then kicked in the front door and shattered the glass.

Pring-Wilson testified Colono and Rodriguez both pounded him relentlessly in the head, and he pulled out his folding knife because he was afraid they would kill him.

The fight between Pring-Wilson and Colono broke out as Pring-Wilson walked by a car Rodriguez and Colono were sitting in as they waited for a pizza order. Pring-Wilson said he approached the car because he heard someone call to him and thought they needed directions.

But Rodriguez said Pring-Wilson pulled open the car door and started the fight after Colono ridiculed him. Colono was stabbed five times in the chest and abdomen.

The prosecution focused on the lies Pring-Wilson acknowledged telling police during a 911 call he made seconds after the fight ended, and during police interviews the next morning. He initially said he had witnessed a young man being stabbed, but described himself as a bystander.

Pring-Wilson's attorney, E. Peter Parker, said the deadlock showed that at least some jurors rejected the prosecution's claim that Pring-Wilson was the aggressor.

"We are thrilled that a number of jurors at this trial saw the commonwealth's case for what it was, and found that Alex's conduct was a justified act of self-defense," Parker said.

Colono's brother, Marcos, and mother, Ada, did not immediately return a call seeking comment.