(CNN) British authorities examining the poisoning of former Russian spy Sergei Skripal believe the daring placement of the nerve agent on his door shows a sophistication that likely had the approval of the Kremlin, a source briefed on the investigation has confirmed to CNN.

Skripal, 66, and his daughter Yulia, 33, were poisoned on March 4 after being exposed to what the British government says was a military-grade nerve agent.

British officials believe an attack of this type could only be carried out by a person (or more than one person) with key training involving Novichok, the substance used in the attack.

There are doubts that rogue agents could have carried out the attack without approval from the top levels of Russian government.

Police officers stand outside the home of former Russian spy Sergei Skripal following his alleged poisoning.

The New York Times first reported this latest theory regarding responsibility.