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 April 12, 2003.

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