MOSCOW — Before the authorities in Britain identified Ruslan Boshirov and Aleksandr Petrov as assassins sent by the Russian government to poison a former spy living in England, men with these names traveled several times within Europe, worked in the pharmaceutical industry and kept social media profiles.

But the portraits sketched by public records and social media are very thin, and even in places where people would be expected to know them, no evidence has emerged that anyone did. British officials say the names are probably aliases.

The meager details of two lives sketched from records reported by Russian news organizations could show the real biographical information of people who just did not leave much of a trail, cases of mistaken identity, or the meticulous work of a spy agency creating a cover, a “legend” in espionage parlance, for agents before a mission.

British officials contend that the two men used a powerful agent known as Novichok on March 4 in an effort to assassinate Sergei V. Skripal in Salisbury, England. Mr. Skripal, a former Russian intelligence officer, had secretly fed information to Britain, was imprisoned in Russia and was then sent in 2010 to live in England as part of a spy swap.