LONDON – Prime Minister Theresa May said Monday that British investigators had concluded it was “highly likely” that Russia was responsible for the poison attack that left a Russian double agent and his daughter comatose on a park bench last week.

The British leader said the police had had identified the poison as a “military-grade nerve agent of a type developed by Russia.”

She warned that Russia had either engaged in a direct attack against Britain or has lost control of the nerve agent it developed. Britain will not tolerate such a “brazen attempt to murder innocent civilians on our soil,” she warned.

As she addressed the House of Commons, the British leader stopped short of announcing retaliatory actions, saying that she will give Russia a chance to respond to her government’s findings and will return to Parliament on Wednesday with a plan for specific action.

In her remarks, May described the assault as a “reckless” and “indiscriminate,” not only endangering the lives of its two principle victims, Sergei Skripal, 66, and his daughter Yulia, 33, but potentially exposing scores of others to the nerve agent.

Skripal was jailed in Russia in 2006 for selling state secrets to the British intelligence services but was released in 2010 as part of a high-profile spy swap. His daughter still lives in Russia, but has also spent long periods in England.

British authorities cordoned off a restaurant and pub near where the couple were found in downtown Salisbury, a quiet medieval town in south England, best known for its nearby ruins, Stonehenge.

Over the weekend, days after the initial attack on March 4, British public health officials advised anyone who had patronized the businesses during a two-day period to wash their clothes, double-bag articles for dry cleaning and wipe down items such as jewelry.

During her question-and-answer session in Parliament on Monday afternoon, members of May’s government and the opposition took turns denouncing the attack as a “murderous,” brazen assault “without impunity” by “Russian mafia state.”

May promised it would not be “business as usual,” but that by Wednesday her government would offer up detailed measures, depending on what the Russians said.

In her remarks, May revealed that British investigators had concluded the chemical used in the attack was part of a group of Russian nerve agents known as “Novichok.”

“Based on the positive identification of this chemical agent by world-leading experts at Porton Down, our knowledge that Russia has previously produced this agent and would still be capable of doing so, Russia’s record of conducting state-sponsored assassinations, and our assessment that Russia views some defectors as legitimate targets for assassinations, the government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal,” she said.

Foreign Secretary Boris Johnson was instructed to demand that the Moscow “immediately provide full and complete disclosure” of the Novichok program to the Organisation for the Prohibition of Chemical Weapons.

President Vladmir Putin’s government has strongly denied any involvement in the case and has responded by accusing Britian of of stirring anti-Russian hysteria.

A BBC reporter quoted Putin Monday as saying, “Get to the bottom of things there, then we’ll discuss this,” when asked about the alleged poisoning.

Before May’s address, the Russian Embassy in London said, “we are outraged by the anti-Russian media campaign, condoned by the government, that influences the investigation and has a psychological effect on British residents.”

In its statement, Russia warned that the British government was playing “a very dangerous game” with public opinion and that this “unhelpful political track . . . also bears the risk of more serious long-term consequences for our relations.”

The Russian said that Russian nationals and others living in Britain are worried about their future there and that Russian journalists based in the U.K. are receiving threats.

On Monday a popular TV anchor on Russian state TV accused Britain of masterminding the poisoning of the former spy and his daughter to undermine Russia as it prepares to host of the soccer World Cup games this summer.

Dmitry Kiselyov, the broadcast journalist suggested it was all a plot to ruin the games for Russia and get British allies to boycott the World Cup.

“Why not poison him?” said Kiselyov. “Is he so valuable? And do it with his daughter to turn it into a real tear-jerker for the public.”

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