Russia on Tuesday dismissed accusations of any involvement in the poisoning of an ex-spy and his daughter as "nonsense," saying it will only cooperate with a British investigation if it receives samples of the nerve agent believed to have been used.
Police, meanwhile, said the investigation of who poisoned Sergei Skripal and his daughter, Yulia, will last many weeks and that they are not ready to identify any people of interest in the inquiry. The father and daughter remain in critical condition in a Salisbury hospital.
British Prime Minister Theresa May said Russia's involvement is "highly likely," and she gave the country a deadline of midnight Tuesday to explain its actions in the case. She is reviewing a range of economic and diplomatic measures in retaliation for the assault with what she identified as the military-grade nerve agent Novichok.
U.S. and European officials were quick to offer words of support for Britain, which will need the backing of its allies if any new sanctions are to have any impact.
Her Downing Street office said that she discussed the Salisbury incident with President Trump, and that the U.S. was "with the U.K. all the way" in agreeing that Russia "must provide unambiguous answers as to how this nerve agent came to be used."
They also agreed on the need for "consequences" for those who use "heinous weapons in flagrant violation of international norms," the White House said.
Earlier, Trump had said: "It sounds to me that they believe it was Russia and I would certainly take that finding as fact."
Russian Foreign Minister Sergei Lavrov told reporters in Moscow that his country's requests to see samples of the nerve agent have been turned down. He insisted that Russia is "not to blame" for the poisoning.
"We have already made a statement to say this is nonsense," he said. "We have nothing to do with this."
The Russian Embassy in London tweeted that it will not respond to the ultimatum without the samples.
Russian officials and media have responded with a variety of accusations against Britain in recent days, including suggestions that it was seeking to influence Sunday's election, which President Vladimir Putin is expected to win easily.
James Nixey, head of the Russia program at the Chatham House think tank, said May's response must be more than symbolic.
"Will actions meet with responses which have real-world effects?" he said. "Or are we going to have more fudge?"
Conservative lawmaker Tom Tugendhat, who chairs the House of Commons Foreign Affairs Committee, said financial sanctions would be keys to a strong response.
"Given that the regime is built on money — it's effectively a flow of money from the Russian people to Putin and from Putin to his acolytes — money matters," he said.
"We have enormous amounts of control of a lot of people's assets through various means, and I think it's important we exercise that," Tugendhat said. "If you get the right people and you freeze their assets, it can make a lot of difference."
The cases of other Russians who have died under mysterious circumstances also are being raised. British Home Secretary Amber Rudd said police and the domestic security service will look into 14 deaths in Britain that might be linked to Russia.
"In the weeks to come, I will want to satisfy myself that the allegations are nothing more than that," Rudd said. "The police and MI5 agree and will assist in that endeavor."
BuzzFeed News reported in 2017 that 14 deaths in Britain and the U.S. dating to 2006 may have been linked to Russia. Among them are prominent Putin critics, including oligarch Boris Berezovsky and whistleblower Alexander Perepilichny.
The chief of the world's chemical weapons watchdog also said that those responsible "must be held accountable."
Skripal, a former Russian military intelligence officer, was convicted of spying for Britain and then released in a spy swap. He had been living under his own name in Salisbury for eight years before the attack without attracting any public attention.