The British government says that both were hurt due to "exposure to a nerve agent". Speculative media reports talk of Sarin and VX, two deadly nerve-agents used in military chemical weapons. Anonymous officials strongly hint that 'Russia did it'.
New reports though point to a deep connection between the case and the anti-Trump/anti-Russia propaganda drive run by the Obama administration and the Hillary Clinton election campaign.
Sergei Skripal once was a colonel in a Russian military intelligence service. In the early 1990s he was recruited by the MI6 agent Pablo Miller. He continued to spy for the Brits after his 1999 retirement. The Russian FSB claims that the British MI6 paid him $100,000 for his service. At that time a Russian officer would only make a few hundred bucks per month.