On Friday morning, someone attempted to detonate a bomb on a London underground train at Parsons Green station. At least 22 people were wounded and taken to the hospital, though none are in a serious condition.

I believe the attack was likely carried out by a relatively unskilled one or two-person cell, inspired but not directed by the Islamic State (ISIS) or al Qaeda (whose chief bombmaker recently called for train attacks).

First off, it's worth noting that the improvised explosive device failed to detonate. The burn-centric nature of the injuries, rather than shrapnel and concussive force centric, suggests that only the detonator ignited, and that the main explosive device remained inert. That's good news because the device, contained in a large paint bucket, was likely packed with shrapnel and explosives.

That said, the available indications suggest the attack was pre-planned. In part, that's because Parsons Green is a top rush hour location in London. I used to travel its route to college for a number of years (my mother still uses that route three or four times a week) and can attest that those trains are always packed at 8:20 in the morning. The attacker was likely well aware this location offered a maximum chance of casualties. Parsons Green is also outside of central London, so the response time of the U.K.'s counter-terrorism response forces would have been slightly slower.

Still, based on Britain's quick decision not to raise the terror alert level, I believe the cell is likely small in number and inspired, rather than directed, by ISIS or al Qaeda. Had they suspected the cell was larger or in possession of more bombs, British authorities would have raised the terror alert level in fear that other attacks are imminent. That they have not suggests they are confident they know who the attacker is, expect to detain him or her soon, and believe a broader operational cell is not involved.

On the inspiration issue, it's also important to note that constructing a viable explosive device is complicated and requiring of either skill, personal tutelage, or access to a reliable guide. But seeing as the British authorities effectively monitor those who seek to build bombs by, for example, googling the method, most terrorists get caught before they can strike. In addition, Britain's GCHQ (NSA equivalent) is also skilled at monitoring ISIS communications between its officers in Syria and Europe. Had the authorities known of any such communication, the party responsible for this attack would have been placed under 24-hour surveillance and prevented from getting to Parsons Green. That the attacker didn't attempt to blow him or herself up is also suggestive of a lower-level training (suicide bombers are the al Qaeda/ISIS preference).

Then there's Trump's tweet from this morning, that British authorities knew the attacker/s identity.

Due to the very close intelligence relationship between the U.S. and Britain, Trump is briefed on all the available investigative information as soon as it becomes available. We must assume the president is thus correct, and that the individual/s involved were known to authorities. Yet Trump's follow up rebuke on the need to be "proactive" is silly. As I've explained, the number of jihadists who pose high-level threats is around 3,000, but it is also joined by a growing threat from Northern Irish terrorist groups. That reality requires prioritizing resources to the highest risk targets: the most skilled and compartmented cells. Today's attacker obviously didn't fall into that category.

Regardless, now stacked alongside a summer of other terrorist attacks, today's events prove that the European terrorist threat isn't going away.