UK intel: Russia tested nerve agent on door handles before Skripal attack

In a letter to the NATO secretary general, Jens Stoltenberg, UK national security adviser Mark Sedwill says there is "no plausible alternative explanation" for the attack in Salisbury on March 4 that left the Skripals hospitalized.

Yulia was discharged from the hospital earlier this week and is being kept in an unknown location. Her father remains hospitalized but is no longer in critical condition.

Russia has denied all involvement in the incident, which the UK blames on Moscow.

Police believe the pair first came into contact with the nerve agent, identified by the Organisation for the Prohibition of Chemical Weapons as Novichok, at Sergei Skripal's home and identified the highest concentration of the substance on the front door.

Sedwill outlines how the Soviet Union "developed a new class of 'fourth generation' nerve agents, known as Novichoks" during the 1980s and Russia was probably the only former Soviet republic to pursue "an offensive chemical weapons programme after independence."

He goes on to describe how Russia later launched a program to test methods of delivering chemical warfare agents and to train personnel in their use.

"This programme subsequently included investigation of ways of delivering nerve agents, including by application to door handles," the letter says.

Regarding the Skripals themselves, Sedwill claims that email accounts belonging to Yulia Skripal were targeted by Russian intelligence agents in 2013, "indicating Russian intelligence service interest in the Skripals."

"It is highly likely that the Russian intelligence services view at least some of its defectors as legitimate targets for assassinations," Sedwill writes, alluding to a possible motive for the attack on Sergei Skripal, a former Russian military official convicted of being a British spy.

He concludes: "We therefore continue to judge that only Russia has the technical means, operational experience and motive for the attack on the Skripals and that it is highly likely that the Russian state was responsible. There is no plausible alternative explanation."