Giampietri was one of the naval officers implicated in the massacre on El Frontón, a prison island off the coast of Callao. The massacre took place during Alan García's administration, on June 18, 1986, after Shining Path prisoners staged an uprising at El Frontón and two other prisons. All the prisoners involved in the rebellion were killed, and Human Rights Watch claimed that evidence suggested that "no fewer than ninety" of the prisoners killed were victims of extrajudicial executions.