The world will most likely never know the precise origins of the deadly nerve agent used to poison former Russian double agent Sergei Skripal and his daughter, Yulia, in the English city of Salisbury last month, says a former Russian chemical engineer who worked on developing the Novichok family of poisons.

"Imagine that you have the corpse of a murder victim," chemist Vladimir Uglyov told Current Time TV, the Russian-language network run by RFE/RL in cooperation with VOA. "The criminal has left clues, including fingerprints. But you don't have those fingerprints in your database. That means you can only identify the killer if you catch him."

The source of the poison used in the Skripal case, Uglyov said, is almost certainly "some beaker standing in some safe in some city in some country."

British officials and allied governments have blamed Moscow for the March 4 poisoning of the Skripals, which has prompted sanctions and diplomatic expulsions and threatens to further harm relations between Russia and the West. Moscow has maintained its innocence.

Although Uglyov believes U.K. authorities' assertion that a nerve agent called A-234 was used against the Skripals, he said he is puzzled by the lack of the basic symptoms of A-234 poisoning -- uncontrollable urination and defecation. Such symptoms, he said, should appear if a person is exposed to something approaching half the median lethal dose.

"Maybe the dose was too small, as happened to me," he said.

Uglyov described working in the secret Russian chemical-weapons laboratory in the closed town of Shikhany in Saratov Oblast in the 1990s when the back of his hand was exposed to a small amount of a related substance, A-242. Unlike A-234, A-242 is a granular solid.

"For me, there was a lesion on the back side of my hand that lasted for five or six years," Uglyov said. "Of course, I was afraid because I knew....I was saved only because it was A-242, which crystallized almost immediately.... I didn't have any other symptoms."

Uglyov said that when he realized he'd been exposed, he soaked his hand in a solution of hydrochloric acid and then rinsed it with hydrogen peroxide.

"Then I just washed it with soap and water," he said.

In the Skripal case, British media have reported the nerve agent was used in the form of a gel. Uglyov told Current Time that Novichok normally has a low viscosity, only slightly thicker than water. Adding a "thickening agent" would make it adhere longer to a surface -- such as a doorknob, as has been theorized -- without evaporating, he said.

"The degree of infectiousness is higher in the pure substance than in its gel form," he said. "The gel form would likely penetrate the skin worse, because it has some sort of internal structural network, in a manner of speaking."

"Most likely, the father [Sergei Skripal] touched the door handle first," Uglyov speculated. "The daughter [Yulia] probably didn't touch the door handle herself but probably either brushed it lightly or he touched her with his [exposed] hand. It would appear that he was the primary source and the daughter had secondary exposure. Maybe that's what happened."

Uglyov expressed his certainty that Novichok was used in the unsolved 1995 killing in Moscow of banker Ivan Kivelidi and his secretary, Zara Ismailova. They both died after being exposed to an unknown nerve toxin that had been placed on the mouthpiece of Kivelidi's office telephone.

"When Kivelidi was poisoned, my guardian angel from the KGB or the FSB -- at that time, the KGB already no longer existed and the FSB (Russian Federal Security Service) hadn't really been created -- approached me," Uglyov said. "This colonel had been contacting me over the previous two or three years. As soon as Kivelidi was poisoned, he questioned me. He said, 'We have identified this substance, Vladimir Ivanovich; you made it.'"

Uglyov told the BBC recently that "the logic of events suggests it was Russia" that poisoned the Skripals, "as does the way our leaders reacted."

Officials in Moscow and other Russian sources have variously sought to parry the British accusation of Russian state involvement in the Skripals' poisoning, including saying neither Russia nor the Soviet Union conducted any research "under the direct or code name of Novichok," that countries including Great Britain, the Czech Republic, or Sweden could be sources of Novichok, and with Russian President Vladimir Putin suggesting that Russia has "no such" weapon and that any military-grade nerve agent would have killed the Skripals on the spot.

Speaking to Current Time TV, Uglyov rejected the claims by fellow Russian chemical-weapons engineer Vil Mirzayanov that as much as 10 tons of Novichok was produced at various labs, including in Uzbekistan. Uglyov said the conditions for producing Novichok do not exist in that post-Soviet republic or even in other laboratories in Russia.

"I would estimate that we produced no more than 100 or 200 kilograms of all types [of Novichok] -- A-230, -232, -234, and -242," he said. "Not more than 200 kilograms altogether. And no one else was producing it. Not in Volgograd and especially not, as Mirzayanov has written, somewhere in Uzbekistan."

Yulia Skripal has been released from the hospital and, while doctors have upgraded his condition from "critical," Sergei Skripal remains in a Salisbury hospital.

British officials have said Russian agencies spied for years on the Skripals, and U.S. Defense Secretary Jim Mattis has suggested Moscow sees covert operations like the poisoning as a way to divide the West over who is responsible.