The Burmese Python is spreading throughout south Florida. This powerful constrictor was imported from Asia. Through accidents and carelessness the snakes were released into the wild.

They are invasive. Facing few predators (big cats and alligators and man only) they easily become the top diner on the food chain. They prey on smaller, unwary animals by literally squeezing the life out of them. The effects on native species are devastating. Fortunately the colder winters of northern Florida act as a barrier to future population growth.

America has long had its own slithering reptilian predators: the Rattlesnake, the Cotton-Mouth, and worst of all – the D.C. Tax Constrictor. These, it would seem, are snakes enough.

Similarly, during the time of the rise of the Pythons, another kind of invasive, non-native snake has come to America. That would be the ISIS (or related) terrorist invaders. They have spread far and wide, unchecked by the climate. They face few natural enemies. And they kill through the constriction of fear, bombs, and blades.

Yesterday the New York Times asked why the FBI did not stop the recent spate of terror attacks – Orlando, New York, New Jersey, Minnesota, etc. – of which it had some knowledge.

Why wasn’t the F.B.I. able to prevent the attacks by arresting those now believed to be responsible?

None of the people in those cases had broken the law or given a clear indication that they intended to kill. The F.B.I. is not allowed to conduct open-ended investigations without justification. Traveling to Pakistan or Afghanistan, or expressing admiration for Osama bin Laden or the Islamic State, could put someone on the F.B.I.’s radar, but none are illegal. The F.B.I. cannot jail someone without evidence of a crime and must follow extensive guidelines that are intended to protect privacy and civil liberties.

James B. Comey, the F.B.I. director, is fond of reminding the public of the F.B.I.’s checkered past and what happens when the rule of law is not followed. He keeps a copy on his desk of then-Attorney General Robert F. Kennedy’s approval to wiretap the Rev. Martin Luther King Jr., a move that later brought shame to the bureau.

What can the F.B.I. do to prevent an attack?

A lot, in some cases, but that is not always enough. After Omar Mateen fatally shot 49 people in an Orlando nightclub in June, Mr. Comey revealed that the F.B.I. had investigated Mr. Mateen beginning in 2013. At that time, the bureau opened a preliminary investigation after Mr. Mateen told co-workers he had family ties to Al Qaeda, was a member of Hezbollah and wanted to die a martyr. The F.B.I. used confidential informants to determine whether Mr. Mateen was a terrorist and placed him on a watch list.

A preliminary investigation has a limit of six months, but it was extended another four in Mr. Mateen’s case. But the bureau failed to find evidence that he was plotting an attack or had connections to an overseas terrorist group. In an interview with reporters after the Orlando shooting, Mr. Comey said about the case: “We are also going to look hard at our own work to see whether there is something we should have done differently. So far, the honest answer is, I don’t think so.”

In a failed plot last year to attack an exhibition of cartoons depicting the Prophet Muhammad in Garland, Tex., one of the attackers was already under full investigation. Yet the F.B.I. was unaware that the attackers had obtained guns and traveled across the country. The police killed both men before they carried out their plan.

The plain answer is the FBI doesn’t stop these attacks because it can’t. There are legal constraints on agents. The agency also wastes huge amounts of money and manpower policing plants and business people. Then there’s the fact that the D.C. snakes keep their slimy ISIS cousins coming in and in growing numbers.

Donald Trump’s son metaphorically made reference to the problem in terms of Skittles. In a bowl of”good” candies, three are known to be poisoned. Would you take a handful? I wonder if the ratio isn’t reversed. Maybe a few good apples don’t save the rotten barrel. Or shouldn’t. This year we’ve imported third-worlders by the tens of thousands. The lunatics in D.C. would like to raise that to hundreds of thousands next year. Hundreds of millions are available. The FBI can’t keep pace with the increases.

The Burmese Python is the least of these problems. They are simply God’s creatures, misplaced but desiring food and little else. The terrorists, while very dangerous, are only products of the times (and ignorant hatred). Serpentes Washingtonius, a Satanic creation bent on destruction, is the worst by far.

They say it is best to run them over while hitting the brakes.