As far as I know he was there, and admitted full responsibility for it

"Abimael Guzmán told the Truth and Reconciliation Commission “We, doctors, reiterate that we will not avoid our responsibility [for the Lucanamarca massacre]. I have mine, I’m the first one responsible, and I will never renounce my responsibility, that wouldn’t make any sense.”[

He did and he admitted it

Remember when Fidel proved you don’t have to boil peasants alive to win a people’s revolution?

I wonder why Guzman couldn’t gain the same kind of popular support…