Britain has accused Russia of poisoning Mr. Skripal, and on Wednesday it announced the first charges in the case, singling out two G.R.U. intelligence officers. In making their case, British investigators released security camera images tracking the two Russians to the scene of the crime, and then back to Moscow.

On Thursday, the leaders of the United States, France, Germany and others released a statement expressing “full confidence” in Britain’s assessment that the attack on Mr. Skripal had been carried out by the two officers. Speaking at the United Nations Security Council, Britain’s ambassador, Karen Pierce, raised the possibility of additional sanctions on Russia, which has vociferously denied any involvement in the poisoning.

The findings of the British investigation, particularly the involvement of two G.R.U. officers, suggest that the poisoning was carried out as an act of retribution by Mr. Skripal’s former colleagues.

But retribution for what, exactly?

Six months after the poisoning, a clear picture of the motive remains elusive. Was it a purely symbolic attack, a warning to other Russian operatives to remain loyal? Or did Mr. Skripal do something specific to anger his former comrades?

In the years after his resettlement in England, Mr. Skripal lived openly in his adopted city of Salisbury, drinking at the local pubs and grilling sausages in his yard.

But he traveled to Prague in 2012, where he spent a boozy lunch with Czech intelligence officers. And he went to the Estonian capital, Tallinn, in 2016 to brief local spies. On each of the trips, which were organized and approved by the British foreign intelligence service, MI6, he shared insights into Russian spycraft and possibly contributed information that led to the expulsion of undercover operatives.

“It contributed to improving our work,” said a European official with knowledge of the meetings.

Still, Spain is a special case.