LONDON — Last fall, President Vladimir V. Putin summoned a Kremlin television crew to his residence for a ceremony marking the destruction of Russia’s last declared stocks of chemical weapons.

The occasion called for a touch of theater: Shells were dismantled on camera, decorated with flowery Cyrillic script reading, “Farewell, chemical weapons!” Mr. Putin spoke proudly of Russia’s status as a peacemaker, and derided the United States for lagging behind. An official from the Organization for the Prohibition of Chemical Weapons, the global body that monitors agreements to whittle down stockpiles, stood by, beaming.

Just six months later, Russia has been accused of secretly producing a strain of lethal nerve agents for years, in what would be a grave violation of its international commitments.

A team of inspectors from the global watchdog organization — the same agency that celebrated with Mr. Putin last September — this week joined the investigation into the poisoning of Sergei V. Skripal, a former Russian spy, and his daughter, who were found unresponsive in the English city of Salisbury on March 4.