The military-grade nerve agent that investigators believe was used to poison a former Russian spy and his adult daughter was identified as Novichok, a hard-to-detect Soviet-engineered killer that's nearly 8 times more potent than VX.

Sergei Skripal, 66, and his daughter, 33-year-old Yulia, remained hospitalized and in critical condition Tuesday, more than a week after they were found unconscious on a park bench in Salisbury on March 4.

British Prime Minister Theresa May said Monday that Novichok was first developed by the Soviet Union near the end of the Cold War.

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Its name is roughly translated to “newcomer,” “newbie” or “new boy" -- and it's considered among the most-deadly nerve agents ever made.

According to the BBC, defense officials from the United States traveled to Uzbekistan in 1999 to help dismantle and decontaminate one of the former Soviet Union’s largest chemical weapons testing facilities, a location a senior defector said had been used to produce and test small batches of Novichok.

According to NPR, Novichok is made with organophosphates, commercially available chemicals that are used in fertilizers and pesticides.

Gary Stephens, a pharmacology expert at the University of Reading told the BBC that Novichok is “a more dangerous and sophisticated agent than sarin or VX and is harder to identify.”

Novichok is reportedly 5 to 8 times more toxic than VX, which is the chemical that was used to kill the half-brother of North Korean leader Kim Jong Un last year.

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Making the nerve agent harder to detect, it can be produced in liquid form or as an ultra-fine powder. This also makes it easier to transport and store.

"One of the main reasons these agents are developed is because their component parts are not on the banned list," Stephens said. "It means the chemicals that are mixed to create it are much easier to deliver with no risk to the health of the courier."

Like other nerve agents, Novichok attacks the nervous system, sending blocking messages to muscles. The effects of the poisoning on the human body can be almost immediate, with symptoms reported to appear as quickly as 30 seconds to three minutes following exposure.

Symptoms include constricted pupils, convulsions, drooling, and in serious cases, coma, respiratory failure and possible death.

“The reason you die from these [chemicals] is very simple,” Dr. Lewis Nelson, chairman of emergency medicine at Rutgers New Jersey Medical School, told LiveScience. “If your muscles don’t work, you can’t breathe. And if you can’t breathe you eventually die.”

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British officials issued what they said were precautionary warnings to the hundreds of people who have visited pubs and restaurants in the city of Salisbury since Skripal and his daughter were sickened last week.

"We have now learned there has been some trace contamination by the nerve agent in both the Mill Pub and Zizzi restaurant in Salisbury," Sally Davies, the chief medical officer for England said. "I am confident this has not harmed the health of anyone who was in the Mill pub or Zizzi's."

The health agency added that any clothing should be washed in "an ordinary washing machine using your regular detergent at the temperature recommended for the clothing."

It also said to "wipe personal items such as phones, handbags and other electronic items with cleansing or baby wipes and dispose of the wipes in the bin."

Russia has insisted it is “not to blame” for the poisoning of one of its former spies on British soil.

The country’s foreign minister said Moscow is willing to cooperate with the investigation if it receives samples of the nerve agent, and suggested London would be "better off" complying with its own international obligations "before putting forward ultimatums."

Russian news agencies reported that the Foreign Ministry on Tuesday summoned the British ambassador in Moscow over the poisoning.

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 the small city of Salisbury for eight years before the attack without attracting any public attention.

Fox News' Travis Fedschun and the Associated Press contributed to this report.