Charlie Rowley does not remember exactly where he picked up the box containing the small glass bottle, but he does recall what was inside. It was definitely not perfume as he had originally supposed, but an oily substance that had very little smell at all.

It was only when he regained consciousness weeks later in a British hospital that the police told him what the substance was: a military-grade nerve agent known as a Novichok that British authorities believe was most likely left behind by Russian assassins.

“I was in complete shock when they told me it was Novichok,” Mr. Rowley said in an interview with the British television channel ITV on Tuesday, his first public remarks since he and his girlfriend, Dawn Sturgess, fell ill on July 1 in the town of Amesbury in southern England. Ms. Sturgess died a week later.

British authorities say the nerve agent is the same substance that was used to poison a former Russian spy, Sergei V. Skripal, and his daughter, Yulia. Investigators believe Mr. Rowley and Ms. Sturgess were most likely exposed accidentally when Mr. Rowley picked up the container it was carried in and took it home.