It says 400 witnesses have already given statements.
The victims of the Salisbury attack, meanwhile, remain in a critical but stable condition in hospital, while a police officer also remains in a serious condition.
And it also said it would halt the activities of the British Council in Russian Federation in a tough series of retaliatory measures announced after summoning British ambassador Laurie Bristow. He also cast doubt on the possibilities that the nerve agent was sent through the mail or was placed in luggage that Skripal's daughter brought with her from Russian Federation to Britain.
'There is no alternative conclusion, other than that the Russian state was culpable for the attempted murder of Mr Skripal and his daughter, and for threatening the lives of other British citizens in Salisbury, including Detective Sergeant Nick Bailey.
The confrontation escalated after May accused Moscow on Wednesday of an "unlawful use of force" involving a weapons-grade nerve agent and ordered out the largest number of Russian diplomats from London in 30 years.
May said Britain would also introduce new measures to strengthen defences against hostile state activities, freeze Russian state assets wherever there was evidence of a threat and downgrade its attendance at the soccer World Cup in Russia this summer.
The tit-for-tat expulsion follows after the poisoning of a former Russian spy and his daughter in London on March 4.
"We have no dispute with the Russian people, and a very large part of the work of my embassy here in Russia has been, is, to promote those links, those mutually productive links, between Russia and the United Kingdom", he said.
Separately, Russia's ambassador to Britain Alexander Yakovenko said on Friday that Moscow would exert maximum pressure on London in the standof, the Interfax news agency cited him as saying.
The first use of a nerve agent on European soil since World War II is a direct challenge to the Western alliance, days before elections are nearly certain to give Vladimir Putin a fourth term as Russia's president.
Mauricio Pochettino says Tottenham will deserve trophies, not buy them
Lucas has started two FA Cup games for Spurs since his arrival and had scored in the victory over Rochdale in the previous round. And this will end up playing into the hands of Tottenham, who themselves will be without a few key players.
Russia decided the British diplomats must leave Moscow within a week, its foreign ministry said on Saturday, after a meeting with Britain's ambassador to Russia, Laurie Bristow.
"We will always do what is necessary to defend ourselves, our allies and our values against an attack of this sort, which is an attack not only on the United Kingdom, but upon the global rules-based system on which all countries, including Russian Federation, depend for their safety and security", Bristow told reporters after being informed of the expulsions.
Russian Federation also said it was halting the activities of the British Council, Britain's worldwide organisation for cultural relations and educational opportunities, across the country.
It ordered the diplomats to leave within a week.
"This follows the action we have taken, alongside other measures, to dismantle the Russian espionage network operating in the United Kingdom as a effect of the attempted assassination of two people here in Britain using a nerve agent".
Russian Federation is expected to announce the expulsion of some British diplomats in a retaliatory measure.
The British ambassador had been expecting this call for three days.
Britain's foreign secretary accused Russian President Vladimir Putin of personally ordering a nerve agent attack in Britain, ratcheting up tensions on Friday (Saturday NZ Time) in an increasingly global showdown over alleged Russian meddling overseas.