The head of London's police force has acknowledged that security agencies around the world can do only so much to prevent the type of terrorist attacks that have occurred recently in France and Germany.
"It's a real challenge to get enough intelligence because we cannot follow everybody all the time, we have to make rational choices," said Sir Bernard Hogan-Howe, Commissioner of London's Metropolitan Police.
In a briefing with a group of foreign reporters, Sir Bernard commiserated with French and German police who have been grappling with a string of attacks in Nice, Normandy, Munich and Ansbach.
"I think we're all shocked by what we've seen," he said. "If it was us today, I suspect we would have a very difficult challenge. I can only talk about our system, at the moment we are okay. But tomorrow, who's to say? I think it's a very difficult problem."
London hasn't had a major terrorist attack since 2013, when soldier Lee Rigby was attacked and killed while walking on a street near the Royal Artillery Barracks in Woolwich. Sir Bernard credited better co-ordination with security services, improved intelligence gathering and the city's massive network of closed-circuit television cameras. In the past year, he said, police had disrupted eight terrorism plots, including a couple that involved assaults on police and military officials.
Nonetheless, London is on a "severe" terrorism alert, the second-highest level. "The reason we're [at that level] is because we're worried there may be attacks that get through. I think that's the same across Europe," he said.
Identifying and tracking people who could turn into terrorists remains a challenge. At least 800 people from Britain went to Syria in recent years, with many joining the Islamic State and others in the fight against the Syrian government. Roughly 400 have returned to Britain and the police now have to assess their potential threat. They are ranked on a scale of 1 to 4, with 1 being the most dangerous.
Many of those who returned from Syria were legitimate aid workers or IS fighters who became frightened of the conflict, he said. "You could, therefore, regard them as a lower-risk group. But we can't absolutely guarantee that," he added. "They remain a continuing concern."
He had praise for controversial programs such as Prevent, which obliges teachers and others in Britain to report people engaging in radical behaviour. Critics have said Prevent stigmatizes those who have been reported and unfairly targets Muslims. Sir Bernard said that while it isn't perfect, the program can offer help to vulnerable people and families.
Putting guns in the hands of police officers isn't a solution, he added, because that only increases barriers between cops and communities. The Metropolitan force remains one of the few in the world where the vast majority of officers do not carry guns. Of the city's more than 32,000 officers, only 2,100 are armed. However, that number is slated to increase by 600 because of the attacks in Paris last November that killed 130 people.
"Just arming all police is not always the answer," he said. "And our way is to have well-trained specialist officers, well equipped, well led, who we'd be deploying in large numbers to deal with that type of attack."
One of the most effective tools to combat terrorism, and most other crimes, is the city's vast network of CCTV cameras. After rioting in 2011, which spread across several parts of London, police gathered 250,000 hours of camera footage to seek out the culprits. About 800 officers spent a year combing through the material, leading to 5,000 arrests. Of those charged with a crime, 90 per cent "pleaded guilty because [the video footage] was such powerful evidence," he said.
Britons have become so accustomed to the proliferation of cameras in the subway, on buses, across public places and in some taxis that the country has not had a major debate about privacy issues.
Sir Bernard said that is because the cameras were introduced at the local level. "It wasn't the government saying you're all going to have CCTV cameras. This was local authorities saying we want it in a public space, in shopping centres, and buses wanted it," he said, adding that for police work, the cameras are "incredibly powerful."