LONDON -- British Prime Minister Theresa May threatened "extensive" retaliation against Russia on Monday, saying it's "highly likely" the Kremlin was behind the poisoning of a former spy and his daughter in Britain.

"We will not tolerate such a brazen attempt to murder innocent civilians on our soil," May said.

May gave Russia one day to explain how a banned military-grade nerve agent was used to poison Russian double agent Sergei Skripal and his daughter, who were both found comatose on a park bench.

"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom," May said.

Secretary of State Rex Tillerson reacted Monday saying the attack had no justification.

"We have full confidence in the UK's investigation and its assessment that Russia was likely responsible for the nerve agent attack that took place in Salisbury last week," Tillerson stated. "There is never a justification for this type of attack -- the attempted murder of a private citizen on the soil of a sovereign nation -- and we are outraged that Russia appears to have again engaged in such behavior."

Tillerson added: "We agree that those responsible - both those who committed the crime and those who ordered it - must face appropriately serious consequences. We stand in solidarity with our Allies in the United Kingdom and will continue to coordinate closely our responses."

Novichok was made by the former Soviet Union in the 1970s. Its highly toxic, taking affect within two minutes of exposure. That may have prompted Britain's chief medical officer to issue an extraordinary warning to anyone who was in the area to wash their clothes, eyeglasses and cellphones.

"Any items which cannot be washed and would normally be dry-cleaned, should be double-bagged in plastic until further information is available," said Sally Davies.

Residents wondered why it took a week to issue the warning.

"I think it's a little too late. I think there should have been more information quicker," one resident told CBS News.

It's no surprise the barricades are still up outside that restaurant where Skripal and his daughter had lunch. In fact, it's been reported that the table where they sat was so heavily contaminated it had to be removed and destroyed.

Investigators have now extended the no-go zones of the investigation.

Experts we've spoken to say residents who may have been exposed could suffer respiratory problems, but they shouldn't be lethal. Meanwhile, the prime minister has vowed to punish Russia if she doesn't get full answers, while Russia dismissed the British ultimatum as a "circus show."