LONDON—Shooting from a distance may indicate a killer is experienced, knife murders are personal, while blunt objects could point to the unplanned motives of fear, desperation, or rage.

Even in major international incidents like the one playing out in Salisbury in southwest England this week, an awful lot can be determined from the choice of weapon.

Poisoning a Russian double agent exiled in Britain with a lab-made substance? Well, that’s usually an assassination ordered by the Kremlin.

Britain’s police detectives and security services now need to prove it.

Scotland Yard’s counterterrorism detectives are deep into a sophisticated version of classic murder weapon analysis with the help of Ministry of Defence biological weapons experts at the Porton Down military research facility.

This weapon of mass destruction was deployed against a Russian father and his daughter who were spending a leisurely Sunday afternoon in the picturesque cathedral city of Salisbury. The WMD was so powerful that 21 people have been treated for its effects.

The Porton Down facility has been home to Britain’s defense and technology research since reports emerged from First World War battlefields that the Germans had killed 140 British soldiers with chlorine gas in January 1915. Coincidentally, the highly secretive facility is located on the outskirts of Salisbury, just seven miles from where former Russian military intelligence colonel Sergei Skripal and his daughter, Yulia, were found on Sunday.

Samples were being analyzed within hours of the discovery after local police began to feel a physical reaction and officers raced to shut down the areas of contamination. Witnesses reported seeing the victims unconscious, with their eyes rolled back, and foaming at the mouth.

Skripal and his daughter were isolated immediately. About 24 hours after the attack, it was determined that they were suffering from some sort of nerve agent in their system. While Skripal has stabilized, his daughter remains in critical condition; both are being treated in the intensive care unit, along with a police officer who was called to investigate this mysterious illness.

Based on their symptoms and the contamination patterns, scientists who spoke to The Daily Beast are convinced this was a nerve agent attack and not radiation exposure, a cyanide attack, or a biological weapon.

"In these recent cases, the symptoms described like frothing at the mouth, vomiting, convulsions and coma—that's more likely a nerve agent,” said Timothy Erickson, chief of medical toxicology at Boston’s Brigham and Women's Hospital and faculty at Harvard Medical School. Erickson published a paper last year in the journal Toxicology Communications about last the fatal February 2017 attack on Kim Jong Nam, the half-brother of North Korean dictator Kim Jong Un, which used VX—short for “Venomous agent X.”

VX was invented by British biological warfare experts at Porton Down, the very same facility where tests are underway this week.

Sarin and VX—dangerous neurochemicals that disrupt nerve-organ messaging and shut down basic bodily functions—are the most popular of the agents, but others with similar properties do exist.

A senior intelligence source told the BBC that it is believed sarin and VX were not the agents used, posing the question: What was used instead and what can that tell us about the source?

Around World War II, Nazi scientists synthesized an entire "G-class" of nerve agents that not only included sarin, but also soman, cyclosarin, and tabun, variants that also debilitate the nervous system.

They were discovered accidentally while manufacturing pesticides, which can have similar effects on humans, but they remain extremely difficult to produce. Mark Bishop, a chemical weapons specialist in nonproliferation at the Middlebury Institute of International Studies in Monterey, California, said that producing them requires a technical capacity and scientific know-how that isn't possible in many places. "It's tricky," Bishop said. "It requires a pretty high level of expertise for producing chemicals."

Bishop said it was possible but highly unlikely that the Russians had developed a totally new nerve agent. “They're probably making an attempt [to create other nerve agents], but it's tough. There's no real incentive to create a new nerve agent—they already work so well. The only motivation to create a new one would be if they wanted them to not be identified as chemicals or to fly under the radar."

One option that is unlikely but potentially alarming is that Russia has finally succeeded in its Soviet era mission to create a new class of nerve agents referred to as novichoks whose molecules were not detectable through modern lab testing methods. "They tried to keep it a secret, and there's pretty skimpy evidence that it was happening," Bishop cautioned. "But it's an interesting possibility that would point directly to the Russians."

No matter what substance was used, conclusively tracing the orders back to the Kremlin will prove difficult. Alexander Litvinenko was murdered in 2006 by a fatal dose of the radioactive substance polonium-210, which was slipped into his teapot at an upmarket hotel in Mayfair, central London.

After a similar scientific rush to identify the high-tech poison, Scotland Yard detectives were able to follow a literal trail of radioactivity all over London including the hotel rooms where the assassins stayed and even to the Arsenal soccer stadium where they took in a match before the radioactive traces followed them on a plane back to Moscow.

Judging by the rush to secure Skripal’s home, the restaurant where he shared lunch with his daughter, the pub where they retired afterwards, and the hospital where they were treated, it seems there were fears that contaminated footprints were indeed being left along the way.

That 21 people have been admitted to the hospital for exposure to the mysterious substance is also indicative. It's probably what is termed in the chemical warfare community a “persistent agent.” A substance that was not persistent would have dissipated quickly in a wide open area—advantageous in an attack on a population, for example, in which doing so might help armed forces clear out the civilian population and then move through quickly. The police officer, Nick Bailey, who was affected later at second-hand was so severely afflicted that he had to be treated in intensive care, although he is now conscious and talking.

The weapons experts at Porton Down will be examining every molecule and the patterns of the substance’s distribution around Salisbury in the hope that they can find a specific chemical signature that will allow this agent to be traced back to its source.

When Britain finally held an inquiry into the murder of Litvinenko—another former Russian intelligence agent who was exiled in Britain—it concluded that the order to assassinate him “probably” came direct from Vladimir Putin.

Russia simply denied it, and despite a decade of increasingly tense relations, London did not impose any additional sanctions against Moscow. If a similar trail leads back to Moscow this time, the government will be under great pressure to act more decisively.

At least since the late 1950s, the KGB (the Soviet forerunner of the Russian FSB) has been hunting down enemies abroad. The agent Bohdan Stashynsky was dispatched to Munich with a specially constructed poison pen that sprayed a cloud of cyanide gas at right-wing nationalist Stepan Bandera, who had fought against the Soviet Union for Ukraine’s independence, sometimes in league with the Nazis.

Documents smuggled out of Russia by former KGB archivist Vasili Mitrokhin showed that the Soviets also lent a hand to their Bulgarian comrades who carried out the notorious umbrella assassination of the dissident playwright Georgi Markov in 1978. Markov was on his way to the BBC when a pellet of ricin was fired from the modified umbrella into the back of his leg.

An inquest into the death of Alexander Perepilichnyy will reopen in Britain next month. British police initially believed that it was a coincidence that the whistleblower—who was due to give evidence in a huge corruption case against Russia in Switzerland—suddenly dropped dead while out running near his home.

A botanical expert later discovered traces of the bright yellow trumpet-shaped flowers of Gelsemium elegans in his stomach. The plant, which grows only among the foothills and mountains of Asia, has been known as a naturally occurring nerve agent since at least the 1870s when it’s deadly properties were described by Sir Arthur Conan Doyle, the creator of Sherlock Holmes.

More recently, it has been deployed by contract killers and assassins from China and Russia.

A former KGB officer told The Daily Beast that Western assumptions that these deadly concoctions must have been devised and authorized by the state showed a deep misunderstanding of Russia.

“People actually underestimate the level of corruption in Russia—any Russian will tell you that the corruption is so high that you can get anything, anything you want,” said Alexander Vassiliev. “You want polonium? You get it—just pay the money.”

Vassiliev, who became a KGB historian after retiring from the service, said the deaths of Skripal’s wife, son and brother in recent years made this look more like a mafia revenge attack than a Kremlin-sanctioned mission.

“I was a cadet in the KGB spy school exactly at the time when Putin was—we had the same training, we had the same instructors, we had the same textbooks, so I always have an idea about how he is thinking,” he said. “Intelligence services in civilized countries don't do revenge—emotions shouldn't have a place in espionage— it's not like two guys got drunk in Moscow, decided to go to Britain and kill a traitor, it doesn't work like that.”

But Vladimir Putin knows the power of symbols, and it is perhaps relevant that at just this moment he is looking to be reelected president of Russia with as massive a majority as he can muster. His tough guy posturing with nuclear weapons and the not-so-mysterious attempted murder of a man deemed a traitor by his comrades and many of his countrymen, even when Putin denies any connection to it, fits well into his tough-guy campaign.

Skripal was widely thought to have retired after being involved in the biggest spy swap since the Cold War, which saw Anna Chapman, who was caught in New York, returned to Russia among others.

Some British newspaper reports have suggested, however, that he may have still been working either for MI6—his handlers when he was spying from Russia—or freelancing for private intelligence companies including Orbis, which is run by Christopher Steele, who compiled the infamous Trump/Russia dossier.

“Of course, he was a traitor—he committed high treason. In the Soviet Union he would have been executed, definitely,” said Vassiliev. “But you only want to kill someone in espionage if you expect this guy to bring further damage to your country or your intelligence agency.”

Where Vassiliev, the scientific community and the British authorities all agree, is on the brazenness of this attack, which could never have gone unnoticed.

Bishop, the weapons expert in California, said the failure to immediately kill the targets—and incidental poisoning of 21 people—suggested that this was a sloppy job. "Nerve agents are pretty potent, and you don't need a high concentration to kill someone," he said. "It's really surprising that they're still alive. Either it was not a potent nerve agent or it was not administered efficiently or it was impure and the proper concentration was not transferred."

Vassiliev agreed. “Generally it doesn't look like a special service operation because the whole thing was done in the daylight, as far as I understand. On the other hand you can never be sure about it because many things can go wrong, there could have been a mistake—no secret agent is perfect.”

Leading British politicians have stopped short of blaming Russia directly, but have promised retaliation, calling it an “act of war” if proven.

The use of a nerve agent to assassinate an opponent is a blatant violation of the Geneva War Convention, the Chemical Weapons Convention, and various international chemical weapons treaties. Russia has explicitly stated and certified that they had destroyed their stockpile of nerve agents, in compliance with the Organization for the Prohibition of Chemical Weapons.

No independent checks were carried out, that certification was based on Putin’s word.