Once again, an opponent of Russian President Vladimir Putin is the victim of a mysterious attack. Sergei Skripal, a former Russian intelligence officer who has been living in Britain since 2010, lost consciousness on Sunday after being exposed to a then-unknown substance. (His daughter Yulia Skripal, who was with him at the time, is also fighting for her life. They remain in stable but critical condition as of this writing.) On Wednesday, the British police presented a startling conclusion: Both had been poisoned with a nerve agent. And on Thursday, the authorities revealed that another 21 people had been exposed as well.

To be fair, it will be hard to prove conclusively that the Kremlin was behind this latest episode. Yet it is hard to imagine another scenario. Nerve agents — most often used in chemical weapons — are not readily available for use by private individuals, and governments tend to guard them carefully.

The attack on Skripal fits neatly into a broader pattern. Ever since Putin came to power, many of his enemies have died under suspicious circumstances, and toxins of various kinds have often been involved. The most well-known victim was Alexander Litvinenko, another former spy who died in Britain 2006 after he was poisoned with polonium, a highly radioactive substance. (Skripal worked as a double agent for the British, and served 10 years in a Russian prison before coming to the Britain as part of a spy swap.)

Litvinenko was just one of many: The former dissident Yuri Shchekochikhin died after a sudden illness in 2003; the Moscow hospital in which he spent his last days refused to release any records or details about his ailment. The journalist Anna Politkovskaya fell ill in 2004 after drinking tea on a government plane (she recovered, only to be shot dead two years later). Pro-western Ukrainian opposition leader (and later president) Viktor Yushchenko survived a dioxin attack in 2004, though he was horribly disfigured. Opposition leader (and Post columnist) Vladimir Kara-Murza has lived through two poisonings.

A striking number of suspicious deaths have occurred in Britain, the preferred home of Russian exiles. Reporters at BuzzFeed recently published an in-depth report on 14 cases in which Kremlin opponents have died under mysterious circumstances in Britain. Perhaps the oddest was that of Alexander Perepilichny, an former banker who exposed a major corruption scheme within the Putin entourage. Police who examined Perepilichny’s corpse found traces of a toxin in his stomach that bore a strong resemblance to that found in the poisonous gelsemium plant.

It is worth noting that the details often remain murky. It can be hard to positively identify the substances involved. The perpetrators are almost never caught, and the motives are sometimes unclear. (Why, for example, did Skripal’s assassins wait eight years before making an attempt on his life?)

Yet we should keep the fundamental question in sight: Why do prominent Russians keep dying from poison?

Other regimes do not seem to resort to this method very often. (One notable exception: North Korea, which used a nerve agent to kill Kim Jong-Un’s half-brother at a Malaysian airport last year.) The Chinese tend to use subtler methods of intimidation and persuasion (at least when dealing with opponents abroad). The Iranians prefer to use guns.

So why the Russian predilection for deadly substances? One answer has to do with tactics. Slow-acting poisons enable assassins to make their escape before the victim expires, while also allowing for considerable deniability. Find the gun that fired the bullet, and you can often catch the killer. Tracing a toxin back to a specific assassin is often much more difficult.

More importantly, toxins are weapons of terror. Their elusiveness makes them perfect for sowing fear and uncertainty. (The difficulty of nailing down clear diagnoses merely amplifies the effect.) As British parliamentarian Bob Seely perceptively noted, this calculated ambiguity makes these assassinations a natural component of broader Russian strategies to sow fear and confusion among their enemies — a strategy that includes the influence operations used by the Kremlin to create chaos in elections in the United States and elsewhere.

And consider the fact that the Kremlin has so often resorted to substances — such as polonium or nerve agents — that would be almost impossible for non-state actors to obtain. The message seems clear: yes, we’re the ones who did this. Cross us and we will come after you — even if you are on foreign soil. And we won’t just put a bullet in your head; we will make sure you die in slow agony. (And if your daughter happens to be with you, we’ll take care of her as well.) Small wonder that Litvinenko referred to his homeland as a “mafia state.”

It is important to note, again, that the tactic isn’t restricted to ex-spies. Reports of mysterious deaths have become almost routine in Russia these days. We will never know how many of them really involved the Kremlin — but that’s precisely the point. Each time an important figure expires prematurely, Russians — and especially the powerful oligarchs who depend upon the ruler’s favor — are reminded that Putin can’t be crossed.

You would think he would not need to bother. After all, experts are constantly assuring us that the Russian president is hugely popular. If so, why would he feel compelled to intimidate his own citizens with such thuggish means? Perhaps Putin is less sure of his position than he looks.