More than one hundred police offers were searching Friday for whatever may have contained the nerve agent that critically injured two people in a small English town earlier this week, CBS News correspondent Charlie D'Agata reports. Investigators were still trying to establish how the victims were exposed to the lethal substance novichok before they collapsed.

Novichok is the same kind of toxin used in the poisoning of Russian double agent Sergei Skripal and his daughter Yulia four months ago.

CCTV footage showed poisoning victim Dawn Sturgess the day before she and her partner, Charlie Rowley, fell fatally ill. Buying alcohol for a summer's evening, Sturgess was unaware that within 24 hours the pair would be fighting for their lives after being exposed to the deadly nerve agent.

On Friday, Rowley's brother, Matthew, was still struggling to comprehend what happened.

"He's my younger brother. I love him to bits. I don't want anything to happen to him, and yet it has. How would you deal with it? It's hearbreaking. I really is," he said.

What's not known - or what police won't say - is where the couple came into contact with the nerve agent, D'Agata reports. But on Friday, authorities had sealed off six sites, including new barriers around Rowley's house.

Worryingly, the public have been warned not to pick up any foreign objects, though health officials were in no mood to elaborate.

"We are working with the best information that we've got available," one officials said. "And I'd just like to keep reiterating that we've only seen two members of the public come forward with symptoms that require treatments."

But these two members of the public are in critical condition after somehow being exposed to a military grade nerve agent, D'Agata says, and people he spoke to in Amesbury were not reassured.