Mr Martin Rivas allegedly said nobody would leave the group alive
At the trial of ex-Peruvian President Alberto Fujimori, a former death squad gunman has said its members had a party only hours after they killed 15 people.
Pedro Supo said his comrades drank beer and ate cake to celebrate the birthday of their leader, Santiago Martin Rivas.
Mr Fujimori is accused of authorising two massacres, in which 25 people were killed, during a campaign against the Shining Path insurgency in the 1990s.
If convicted, he could face up to 30 years in prison. He denies the charges.
In December, Mr Fujimori was sentenced to six years in jail and fined $92,000 (£45,000) after being convicted on separate charges of abuse of power.
In his testimony at the trial of the former president, Mr Supo described how he and his comrades in the death squad drank beer and ate birthday cake on the beach just hours after their deadly attack on a poor suburb of the capital, Lima.
We sat around a table and saw a newsflash on TV about what happened - we saw that a child had died, then we had a toast for the birthday of [Santiago] Martin Rivas
Fifteen people were killed in the Barrios Altos massacre, one of them an eight-year-old boy.
Mr Supo, the getaway driver, described to the court how they watched the TV report of the attack and saw that a child had died before toasting their leader and singing him happy birthday.
Some members, he explained, expressed remorse and wanted to leave the group.
But he said Mr Martin Rivas insisted nobody would leave the group alive.
The Barrios Altos massacre was a mistake. Poor intelligence meant the death squad burst in on the wrong barbecue, killing innocent civilians.
Mr Supo said he had understood that the objective had been to seize suspected members of the Shining Path rebel group, but as the hour approached he realised that the purpose was to kill them.
He told the court that it was after that night that the group, known as La Colina, really became a death squad.
Mr Fujimori maintains he knew nothing about the killings until after they happened and denies all the charges against him