Russia has categorically denied any involvement in either the Skripal case or the election interference. In their one-on-one meeting in Finland this month, President Trump and President Vladimir V. Putin of Russia said they had discussed the election meddling. Mr. Putin even offered to allow American prosecutors to sit in on interrogations of the 12 intelligence officers charged in the case. But in exchange he asked that Russian investigators be allowed to interrogate William F. Browder, a vociferous Kremlin critic, and Michael A. McFaul, a former American ambassador to Moscow, among others.
It was unclear, though, whether the two leaders discussed the Skripal case, which prompted the Trump administration to expel from the United States 60 Russian diplomats believed to have been working undercover as spies, by far the most expulsions of Russian officials by any country.
Even if suspects are identified in the Skripal case is it unlikely that the British will ever get prosecute them. After the 2006 death of Alexander V. Litvinenko, the last known former Russian intelligence operative to be poisoned on British soil, British officials tried in vain to get their hands on the two prime suspects, Andrei K. Lugovoi, a former K.G.B. bodyguard, and Dmitri V. Kovtun, a Red Army deserter. Despite volumes of evidence, including a trail of radioactive polonium that investigators were able to trace practically to Moscow, Russia refused to hand over the two men. Mr. Lugovoi later became a member of Parliament, giving him immunity from prosecution at home.
Another lingering mystery in the Skripal case is motive. As many skeptics of Russia’s involvement point out, any information Mr. Skripal possessed from his time with the G.R.U. was dated, and it is unknown whether he had access to new intelligence from the agency. He retired from the G.R.U. in 1999 and was arrested by Russian counterintelligence agents in 2004 for espionage. He spent the next six years in prison before suddenly being released early in 2010 as part of a spy swap.
After settling in England, though, Mr. Skripal did not remain idle. He traveled widely, offering briefings on Russian spycraft to foreign intelligence agencies in the Czech Republic, Estonia and possibly others. In one meeting with Czech officials in 2012, he explained to his former foes the intricacies of G.R.U. operations, said a European official with knowledge of the meeting. In the following months, several Russian diplomats were kicked out of the Czech Republic for spying, though the official could not say whether Mr. Skripal’s revelations led to the expulsions.