Updated at 3:15 a.m. ET on Tuesday

British Prime Minister Theresa May says it is "highly likely" that Russia is behind the poisoning of a former Russian spy and his daughter earlier this month in southern England.

Sergei Skripal and his daughter, Yulia, were found collapsed on a bench on March 4 in the city of Salisbury. They remain in critical condition, according to The Associated Press.

"It is now clear that Mr. Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia," May told lawmakers in a statement on Monday. She said the agent has been identified as one of a group of nerve agents called Novichok, which Russia has previously produced.

She added that there are just two explanations for the apparent poisoning — either Russia directly carried out the attack or it lost control of its supply of the deadly nerve agent. The Russian ambassador has been summoned to explain how this happened, May added, and has been asked to give a formal response by Tuesday.

And if that response is not credible, May said, "we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom. And I will come back to this house and set out the full range of measures that we will take in response."

This case has drawn parallels with a Russia-linked death of a defected Russian intelligence officer in 2006, as NPR's Scott Neuman reported. Here's more:

"Skripal had retired from Russia's SVR foreign intelligence service, a successor to the KGB, when he was arrested and convicted in 2006 of working undercover for Britain's MI6. In 2010 he arrived in the U.K. as part of a prisoner swap between Moscow and Washington."

When the BBC asked Russian President Vladimir Putin whether Russia was responsible, he replied: "Get to the bottom of things there, then we'll discuss this."

"Having established that a nerve agent is the cause of the symptoms leading us to treat this as attempted murder, I can also confirm that we believe that the two people who became unwell were targeted specifically," Metropolitan Police counterterrorism chief Mark Rowley said at a news conference last week, Scott added.

Nerve agents are "designed only to kill," Leeds University toxicologist Alastair Hay told NPR. The weapons "work by blocking the message from the nerves to the muscles," which could affect the muscles used to breathe, potentially causing asphyxiation.

"Trace contamination" was found at two Salisbury establishments — The Mill Pub and Zizzi Restaurant — England's chief medical officer announced Sunday, as NPR's Amy Held reported. People in those establishments on March 4 or 5 were encouraged to clean the clothes they were wearing or wipe down belongings.

At the same time, officials stated that there is "no immediate health risk." A policeman who was responding to the incident, Det. Sgt. Nick Bailey, remains in "serious but stable condition," May said Monday.

White House press secretary Sarah Sanders on Monday declined to say whether the administration believed Russia was involved, but said: "We've been monitoring the incident closely, take it very seriously."