Because the nerve agent is so potent, the officials said, the task could have been carried out only by trained professionals familiar with chemical weapons. British and American officials are skeptical that independent actors could have carried out such a risky operation or obtained the agent without approval at the highest levels of the Russian government — almost exactly the same phrase that American intelligence agencies used in October 2016, when they first attributed the hacking of emails from the Democratic National Committee to a team of Russian hackers.

Four weeks after the assassination attempt, British and American officials are turning to the question of whether President Vladimir V. Putin himself was aware of, or ordered, the attack.

They say there is no evidence so far of his direct participation, but the Russian president, a former K.G.B. officer, is skilled at hiding his communications.