I was stationed in PR in the early 60’s. Had a great time, by the way in Old San Juan! But there were serious terrorist factions that bombed military school buses carrying military dependents. Very anti-American groups.
The independistas group was small in numbers but violent. They bombed communication towers and electrical substations. Cuba was supporting them. In fact while I was there, also in the 60’s, LE found a large supply of fire arms, ammo and explosives in a hidden cove on the NE coast deposited by Cuba and for the independistas.