LONDON — Orlando shooter Omar Mateen was on a terror watch list. So was Larossi Abballa, who stabbed a police officer and his wife on Monday in a suburb of Paris.

Despite being known to authorities, both men succeeded in planning and committing their horrific crimes. Why did authorities not stop them?

Numbers provided by U.S., French and British authorities suggest that it is impossible to monitor all suspects on terror watch lists 24/7 — a situation that is unlikely to change.

In 2014 — when the Islamic State spread nearly unhindered, it seemed, in Syria and Iraq — Britain said its security services only had the capacity to physically monitor 50 suspects. In other words: Less than 2 percent of all suspects on Britain’s terror watch list were actually monitored.

Assuming that agencies need at least 20 staffers to observe an individual day and night, one would continuously need more than 60,000 officers to monitor all 3,000 suspects.

However, the total size of the country’s police force is only 150,000, with an additional few thousand personnel working for security service MI5. If it really wanted to make sure that no suspect goes unmonitored, half of all British police officers would have to be taken off the streets — most likely putting the country at much greater risk.

If France wanted to observe all 11,000 terror suspects in the country, it would need its entire police force of 220,000 officers — or more.

The large number of suspects on Europe’s terror watch lists reflects a phenomenon which has caused headaches among European security services, at least since 2014.

Although security agencies considered the terror threat to be extremely high following 9/11, the number of suspects was contained at that time. Whereas most countries had to keep track of hundreds of suspected supporters of radical Islamist violence, this number has rapidly skyrocketed in the last years to several thousands of suspects in most larger European countries.

But the number of terror suspects also reflects other influences: France and Britain’s counterterrorism strategies are vastly different.

Whereas Britain seeks to keep the number of suspects and arrests as low as possible, France’s strategy is based on disruption and large-scale arrests. This might explain why there are four times more individuals on France’s terror watch list than on the British equivalent — although both countries are considered to be comparably threatened by potential attacks.

The exact number of U.S. citizens or lawful permanent citizens who are on a terror watch list is much harder to know. Most sources refer to the federal government’s central Terrorist Identities Datamart Environment (TIDE) which has over 1 million entries — including 25,000 U.S. citizens or permanent residents. U.S. authorities would need approximately 500,000 staff members to observe all suspects day and night.

But in the case of the United States, things get a bit more complicated: The FBI has its own Terrorist Screening Database with 40,000 U.S. citizens or permanent residents. Some of the names are also included in the federal government’s TIDE database.

But the uncertainty regarding the real number of all American suspects on all U.S. terror watch lists changes little about how there are way more suspicious individuals than can possibly be monitored.

Instead of observing most suspects themselves, intelligence agencies and police authorities have to rely on relatives, neighbors or informants to warn of plots. Research has indeed shown that the overwhelming majority of terrorism plots in the United States were prevented because individuals close to the attackers informed authorities on time.

In the case of the Orlando shooter, too, his wife appears to have had some knowledge of his attack plans hours before the shooting started. But she talked to investigators only after it was over.

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