Terrorist organizations leave long tails. Long after terrorist organizations disappear, they leave behind members who went into hiding. Since many of these terrorists participated in murders, there is no statute of limitations (in most nations) for their crimes. So when the terrorists are later (often 30 or more years later) discovered, they are subject to prosecution. Sometimes that is possible because of advances in technology. One example is former (1970s) German terrorist Verena Becker, who was arrested last year. She was a member of the Red Army Faction, a leftist radical terror group that killed 30 people during dozens of attacks. In the 1960s and 70s, leftist terror groups were very popular among college students and young graduates. Their call for revolution went unanswered, and all these groups faded away as members were killed, arrested, or dropped out, and tried to live down this violent period in their lives.
But Verena Becker had left her DNA on a letter the group sent to the media, taking responsibility for one murder. So Becker is being prosecuted, for a nearly four decade old crime. Becker had earlier been caught and sentenced to life for another murder, but was pardoned and released in 1989.
The same thing has happened to terrorists from that period, all over the world. The current Islamic terrorists will leave a similar legacy. But it will be worse. The Islamic terrorists killed far more people, often using much more brutal methods (torture, decapitation). Moreover, the Islamic terrorists have tended to be more boastful about their crimes, and willing to identify themselves. In the Middle East, where most of the killings took place, people have long memories for such crimes. Vengeance killings will still be an option for family and friends of victims, until all the terrorists die of old age, are caught, or simply caught up with.