UPDATE 1:30 p.m.

ABOARD A U.S. GOVERNMENT AIRCRAFT >> U.S. Secretary of State Rex Tillerson cast the poisoning of an ex-spy in Britain as part of a “certain unleashing of activity” by Russia that the United States is struggling to understand. He warned that the poisoning would “certainly trigger a response.”

Tillerson, echoing the British government’s finger-pointing toward Moscow, said today that he didn’t yet know whether Russia’s government knew of the attack with a military-grade nerve agent, but that one way or another, “it came from Russia.” He said it was “almost beyond comprehension” why a state actor would deploy such a dangerous substance in a public place in a foreign country where others could be exposed.

“I cannot understand why anyone would take such an action. But this is a substance that is known to us and does not exist widely,” Tillerson told reporters as he flew from Nigeria to Washington. “It is only in the hands of a very, very limited number of parties.”

British Prime Minister Theresa May said that Novichock, the nerve agent used against ex-spy Sergei Skripal and his daughter, was developed by the Soviet Union near the end of the Cold War. Skripal, 66, was a Russian military intelligence officer before flipping to the British side in the 1990s, going to jail in Russia in 2006 and being freed in an exchange of spies in 2010. Moscow has dismissed the suggestion it was involved in his March 4 poisoning as “a circus show.”

Tillerson, who spoke today by phone with British Foreign Secretary Boris Johnson, said he’s grown “extremely concerned” about Russia, noting that he spent most of the first year of the Trump administration trying to solve problems and narrow differences with the Kremlin. He said after a year of trying, “we didn’t get very far.”

“Instead what we’ve seen is a pivot on their part to be more aggressive,” Tillerson said. “And this is very, very concerning to me and others that there seems to be a certain unleashing of activity that we don’t fully understand what the objective behind that is.”

He said if the poisoning turned out to be the work of Russia’s government, “this is a pretty serious action.”

“It certainly will trigger a response. I’ll leave it at that,” Tillerson said.

Tillerson, whose relationship with Russian President Vladimir Putin dates back to his days as Exxon Mobil’s CEO, has sought to work with Russia on narrow areas where the two countries could find common ground, such as a cease-fire in southwestern Syria that has largely held since last year. But those efforts have had diminishing results. Tillerson’s efforts to persuade Moscow to stop propping up Syrian President Bashar Assad and to pull out of Crimea have yielded little to no progress.

At the same time, President Donald Trump’s critics regularly accuse his administration of failing to stand up to the Kremlin, especially over Russian interference in the 2016 presidential election. Russia hawks in Congress are particularly miffed that the State Department so far has declined to use a new law letting the U.S. slap sanctions on foreign companies or governments that do business with Russia’s defense or intelligence sectors. Those powers took effect in January, but so far nobody has been punished.

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LONDON >> Russia is “highly likely” to blame for poisoning a former spy and his daughter with a military-grade nerve agent, British Prime Minister Theresa May said today, demanding that Moscow give a compelling explanation or face “extensive” retaliation.

May told lawmakers in a strongly worded statement that without a credible response from Russia by the end of Tuesday, Britain would consider the attack on Sergei Skripal and his daughter in a quiet English city “an unlawful use of force by the Russian state against the United Kingdom.”

“There can be no question of business as usual with Russia,” she said, without saying what measures Britain might take.

Russian Foreign Ministry spokeswoman Maria Zakharova dismissed May’s allegations as a “circus show in in the British Parliament.”

Skripal, 66, and his 33-year-old daughter, Yulia, remain in critical condition after being found unconscious March 4 in Salisbury. A police detective who came in contact with them is in serious but stable condition.

May said British scientists have determined that Skripal and his daughter were poisoned with Novichok, a class of nerve agent developed by the Soviet Union near the end of the Cold War.

She said it was “highly likely” the substance came from Russia, and there were two possible explanations.

“Either this was a direct act by the Russian state against our country, or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others,” she said.

May said Britain had given the Russian ambassador in London a deadline of Tuesday to explain which version is true. She said Russia must also “provide full and complete disclosure” of its Novichok program to the Organization for the Prohibition of Chemical Weapons, the oversight body for the international chemical weapons convention.

May spoke in the House of Commons after she chaired a National Security Council meeting to hear the latest evidence in the case. She has been under mounting pressure to hit Russia with sanctions, diplomatic expulsions and other measures in response to the poisoning, the latest in a string of mysterious mishaps to befall Russians in Britain in recent years.

May said Britain would consider tough action if Russia’s explanation is inadequate, though she didn’t give details.

She said Britain would be prepared to take “much more extensive measures” than the expulsions and limited sanctions imposed after the death of former Russian agent Alexander Litvinenko, who was poisoned by drinking tea laced with radioactive polonium in London in 2006.

“We will not tolerate such a brazen attempt to murder innocent civilians on our soil,” May added.

The White House said the use of the nerve agent “is an outrage” but wasn’t ready to say that Russia was responsible.

Spokeswoman Sarah Huckabee Sanders called the poisoning “reckless, indiscriminate and irresponsible,” adding that the U.S. stands by its ally.

British opposition lawmakers are urging the adoption of a version of the United States’ Magnitsky Act, a law allowing authorities to ban or seize the assets of individuals guilty of human rights abuses. It is named for Sergei Magnitsky, a lawyer who died in a Russian prison after exposing a $230 million fraud involving organized crime and a Russian government official in 2008.

Critics of the British government say the U.K.’s response to Russian wrongdoing has been muted because London’s property market and financial sector are magnets for billions in Russian money.

James Nixey, head of the Russia program at think-tank Chatham House, said Britain has for years avoided tough decisions about Russia.

“There has been a lot of tough talk over the years and almost no action to protect our national security and integrity,” he said. “We have sent mixed signals to Russia. We have talked tough by calling it names and expressing our dislike of the regime, but at the same time we have been very much open for business with Russia, of any kind.”

He said May will have a difficult decision on Wednesday, when she has to spell out how Britain will respond.

Alastair Hay, professor emeritus of environmental toxicology at the University of Leeds, said May’s remarks amounted to “saying that Britain has been attacked with chemical weapons.”

Noting that Russia had previously announced it had destroyed all of its stocks of chemical weapons, Hay said May’s remarks raised “a whole new set of questions: Des Russia have another kind of chemical weapons program that it hasn’t declared?”

Many see echoes between the Skripal case and the killing of Litvinenko, which a British inquiry concluded was the work of the Russian state, probably on the orders of President Vladimir Putin.

The Kremlin has denied involvement in Litvinenko’s death, and it dismissed claims it was behind the attack on the Skripals.

Putin spokesman Dmitry Peskov told reporters that Sergei Skripal worked for British intelligence and was poisoned on British soil, and therefore the incident “has nothing to do with Russia, let alone the Russian leadership.”

Skripal was a Russian military intelligence officer when he was recruited to spy for Britain in the 1990s. He was jailed in Russia in 2006 for revealing state secrets before being freed in a spy swap in 2010. He had settled in Salisbury, 90 miles (140 kilometers) southwest of London.

Almost 200 troops, including soldiers trained in chemical warfare and decontamination, have been deployed to Salisbury to assist the police investigation into where the nerve agent came from and how it was delivered.

British officials have said the risk to the public is low, but urged people who visited the Zizzi restaurant or Mill pub, where the Skripals went before their collapse, to wash their clothes as a precaution. Some have questioned why it took health authorities a week to issue the advice.

Andy Harder, 63, who works in a coin and stamp collector’s shop in Salisbury, was in the Mill pub the day after the Skripals fell ill and before police cordoned off the area.

Harder said he washed his jacket off with an antiseptic cleaner after authorities gave the guidance Sunday.

“So I’ve washed all my clothes, I’ve taken my jacket and done that with Dettol — I mean I don’t know what to use, really,” he said, referring to a brand of disinfectant. “That’s supposed to kill most things. I’ve had a good scrub up, so it should be OK.”