Recall that an ex-spy and his daughter were attacked in a British park with a view to killing at least the ex-spy. The weapon used was a nerve gas that’s only made in Russia. The investigation itself is pointing strongly at Moscow as the instigator of the murder attempt; although the investigation is not complete. Now Russia is refusing to cooperate in the investigation unless certain conditions are met.
Russian Foreign Minister Sergei Lavrov said that Moscow would only cooperate with the UK’s investigation into the nerve-agent poisoning of a former spy if London supplied the substance in question and opened up the probe to Russian officials.
Supplying a sample of the nerve agent—which use just incidentally also poisoned roughly 20 other folks associated with the discovery of the two victims, their transport to a hospital, and their treatment, thereby illustrating the indiscriminate nature of the attack—would be entirely reasonable.
That last, though, opening the investigation to the Russians should be unacceptable to the Brits for a couple of reasons. One is that the Russians are unlikely to cooperate with, much less support, the investigation in any serious way regardless of any sharing by British investigators.
The larger reason, though, is that the opening would only provide Putin with another avenue for spying on Great Britain.