LONDON -- President Trump joined British Prime Minister Theresa May in calling for Russia to come clean about the poisoning of a former spy in England. That came as another Russian turned up dead in London. The body of Russian exile Nikolai Glushkov was discovered Monday night, less than two weeks after Russian ex-spy Sergei Skripal and his daughter were poisoned by a military-grade nerve agent made in Russia.

Glushkov was an opponent of Russian President Vladimir Putin, and joins a list of 14 other Russians who have died on British soil -- some under mysterious circumstances -- and whose cases are now being reviewed. Skripal and his daughter Yulia remain in critical condition, and the police detective who was first on the scene is still in serious but stable condition.

Military personnel in protective suits remove a police car and other vehicles from a public car park as they continue investigations into the poisoning of Sergei Skripal on March 11, 2018 in Salisbury, England. Getty

The chemical nerve agent used in the attack was Novichok, a particularly lethal weapon developed by Moscow near the end of the Cold War.

"It's an unbelievably cruel and unpleasant way to go," says toxicologist Andrea Sella. "In terms of the amount, we are literally talking about a few drops here, are sufficient. The real crucial question is how you deliver it."

Sella says Novichok is made by mixing two relatively harmless compounds together, which become instantly lethal. It is usually found as a liquid and can be sprayed or swallowed.

"We speak of these as weapons of terror because they really are," he said.

Meanwhile in Russia, Foreign Minister Sergey Lavrov dismissed the allegations as "nonsense" and demanded the British hand over a sample of the nerve agent.

Russia had until midnight to provide some explanation of how the nerve agent made its way to Britain. It's now up to May to follow through on threats to increase sanctions, freeze assets of Russian oligarchs here, and possibly expel diplomats.