Britain’s top domestic intelligence official issued a stark warning on the 25th anniversary of the September 11 attacks, saying the next major security catastrophe could already be developing beyond the areas intelligence agencies are currently watching.

Ken McCallum, director general of Britain’s MI5 intelligence service, said that while security agencies must continue learning from previous terrorist attacks, concentrating too heavily on yesterday’s threats could leave countries dangerously exposed to an entirely different type of attack in the future.

McCallum made the warning in an opinion piece published Friday to coincide with the 25th anniversary of the September 11, 2001, terrorist attacks, when al-Qaida terrorists hijacked passenger aircraft and attacked the World Trade Center and Pentagon. His remarks were also reported by Reuters.

“We know that the next strategic shock will, by definition, not resemble the last one. That is perhaps the most important lesson of all,” McCallum wrote in the commentary for the Independent newspaper.

The MI5 chief cautioned that intelligence agencies cannot simply prepare to prevent another attack modeled on what happened on 9/11. The next catastrophic threat, he suggested, may emerge through entirely different methods, actors or targets.

“We must of course always learn from the last attack – but must beware of preparing brilliantly to fight yesterday’s enemy, using yesterday’s methods, against yesterday’s target … The uncomfortable truth is that the next strategic shock may already be taking shape somewhere we are not yet looking.”

McCallum stressed that the terrorist threat has not disappeared. Although al-Qaida is substantially weaker than it was at the time of the September 11 attacks, he said the organization remains dangerous, while it and other terrorist groups continue to harbor ambitions of carrying out attacks capable of inflicting mass casualties.

He pointed in particular to a recently thwarted Islamist terrorist plot targeting the Jewish community in Manchester, England. According to McCallum, the plan called for gun attacks intended to murder hundreds of Jews — an example, he said, of the continuing desire among extremists to carry out large-scale atrocities.

But terrorism is no longer the only danger occupying MI5.

McCallum said British intelligence is increasingly confronting threats backed by hostile states. Those operations can take forms ranging from sabotage carried out through criminal proxies to cyberattacks targeting critical infrastructure, including power stations.

That changing landscape, he argued, means governments cannot prepare for the next major crisis on their own.

The world’s infrastructure also looks markedly different from what existed in 2001. Many systems and essential services that were once largely controlled by governments are now operated by private companies. McCallum said protecting those systems will therefore require cooperation between government agencies, private businesses and academia.

The time to establish those relationships, he warned, is before an attack occurs — not while a crisis is already unfolding.

“We cannot wait for the next shock to discover who owns the vulnerability, who has the authority to act, or who was supposed to pick up the phone,” he wrote.

McCallum framed that preparation as part of the responsibility owed to the nearly 3,000 people murdered on September 11, arguing that marking the anniversary cannot be limited to looking backward and remembering those who were killed.

Instead, he said, the lesson of 9/11 must include preparing for threats that may look nothing like the attack that changed the world 25 years ago.

“We owe the victims of 9/11 – and those who will otherwise become the victims of tomorrow’s attack – something more than remembrance. We owe them preparation, resilience and the courage to adapt before the next shock arrives.”