LONDON — The British authorities said on Wednesday that Sergei V. Skripal and his daughter, Yulia, the poison victims at the epicenter of a diplomatic crisis between Russia and the West, had been sickened with a nerve agent on the front door of Mr. Skripal’s house.

The announcement narrows the many possibilities of how the Skripals came into contact with the poison.

Detectives will focus their efforts now on Mr. Skripal’s house in the cathedral town of Salisbury, England, said Dean Haydon, senior national coordinator for counterterrorism policing, in a news release.

They will also, presumably, search for a person who could have delivered the poison.

Mr. Skripal, a former Russian double agent, and his daughter, who had been visiting him from Russia, were found unresponsive on a bench in a Salisbury park on March 4.