President Vladimir V. Putin of Russia insisted in an interview published on Friday by The Financial Times that Russia had not tried to poison Mr. Skripal, arguing that he had already served a sentence in a Russian prison for assisting British intelligence. He did, however, speak with open contempt for traitors in general.

“Treason is the gravest crime possible, and traitors must be punished,” he said, adding, “I am not saying that the Salisbury incident is the way to do it. Not at all. But traitors must be punished.”

British officials identified Mr. Skripal’s attackers as colonels in Russia’s military intelligence service, the G.R.U., who were caught numerous times on video surveillance footage near Mr. Skripal’s home, where traces of the nerve agent was found and which sickened a local police officer.

Bellingcat later revealed their true names as Anatoly V. Chepiga and Aleksandr E. Mishkin. But until now, few details about the “third man” believed to accompanied the team to London have been reported.

The group’s findings suggest that the mission was put together hurriedly.

Mr. Sergeyev, it said, received confirmation that he would have to fly to London at 6 p.m. on March 1, just three days before the poisoning, and then called travel agencies, searching for last-minute flights from Moscow to London. He received a booking confirmation just after 8 p.m., Bellingcat reports, and then searched to see whether he would need a new data plan.