The FBI just arrested an al Qaeda collaborator in Cleveland who wanted to bomb a July 4th fireworks show. Demetrius N. Pitts had declared his intentions to an undercover FBI agent.

Yet when asked at a press conference whether Pitts had the ability to effect his attack plans, the FBI agent in charge stated that "[Pitts'] precise ability to do specific things" might always be unclear. The agent also clarified that the agency doesn't "have the luxury" of waiting to see whether terrorists finalize their attack plans by purchasing explosive precursors or recruiting new suspects to their cause.

This arrest and those comments illustrate the broader U.S. approach to counterterrorism investigation — an aggressive approach that contrasts with more deliberate pace of British officials.

The first difference is when the U.S. and British authorities choose to intervene against a suspect. Where U.S. authorities tend to arrest a suspect when they believe they can prove he/she intended to carry out a terrorist attack, British authorities tend to wait to arrest a suspect until they believe he/she has finalized their attack plans.

Why the distinction?

First, it's because the U.K. authorities face higher evidence standards for terrorism-related convictions. While the British domestic security service, MI5, and its police counterparts have significant surveillance authorities (greater than those afforded to the FBI), they struggle to get convictions with long prison sentences if they can only prove a suspect's intent to do harm. To put a suspect in prison for a large number of years — and thus best protect the public — British authorities must prove both a suspect's intent to carry out terrorist offenses and his/her credible means of doing so.

As an extension, Pitts' arrest also speaks to the FBI's greater counterterrorism resources and comparatively (although still quite a few) fewer suspects than their British partners. Those resources allow the FBI to resource investigations like this one by slowly cultivating knowledge of a suspect's plans while also guarding against a suspect's rapid escalation to the point of attack (a particular Islamic State-related problem). The British authorities, however, must shuffle surveillance resources to those who they believe pose the greatest threat in any one week. Were the British to resource investigations against suspects like Pitts' as the FBI has done, they would overwhelm already stretched British surveillance resources.

There's another factor here: the balance between stopping a risk and gathering intelligence. In terrorism investigations, security services must balance their desire to quickly remove a threat with their desire to fully understand that threat by letting things go on a bit longer. But where the FBI leans toward the immediate removal of a threat, MI5 has a cultural predisposition towards allowing suspects to remain "in play." This allows MI5 to learn whether the suspect has a broader network of direction, support or co-conspirators.

The U.S. and U.K. preferences can come into tension. During al Qaeda's 2006 transatlantic terrorist plot, for example, the CIA and FBI had a major argument with British authorities over when to arrest the suspects. Where the British wanted to keep monitoring the suspects to develop a better intelligence picture of al Qaeda operations in the West, the U.S. wanted to remove the threat. In the end, the British were forced to arrest the suspects when the CIA forced their hands via some antics in Pakistan.

Fortunately, both sides of the Atlantic alliance have excellent counterterrorism services and embedded cooperation. Their activity, especially in the field of boutique signal intelligence capabilities, has prevented many bloody attacks and saved many lives across the world. Still, cooperation doesn't make the FBI and MI5 one and the same.