The sickening scene from Britain of a blood-spattered man spouting Islamist hatred, who had just beheaded an off-duty British soldier in broad daylight, sends shivers down the spine. Is this the face of modern terrorism? If so, is no one safe anymore?

After the initial horror at the barbaric butchery on a leafy London street come questions about our attempts to prevent terrorism. Eleven years on from the attacks of September 11, we are still left grappling with some basic questions: What exactly is terrorism? And what can we do, if anything, to prevent it?

The British prime minister, David Cameron, his colleagues, and top officials and police have been careful not to jump to conclusions. They have avoided the rush to judgment that so many in the United States urged on the Obama administration in describing the motivation of the killers of ambassador Chris Stevens in Benghazi. It is not that they do not take the killers at their word; it is because they simply do not know.

Cameron was careful not to prejudge or speculate on the motives of the two suspected culprits, who made no attempt to escape and waited for the police to come and arrest them. “There are strong indications that it is a terrorist incident” was as far as he would go. It was even left to French President Francois Hollande – Cameron was in Paris ‑ to let slip that the British authorities knew the victim was a soldier.

The Metropolitan Police chief was even more cautious. “We understand concern about the motivation, and we will work tirelessly to uncover why this occurred and who was responsible,” he said. “I understand people want answers, but I must stress we are in the early stages of investigations.”