According to the paper, Skripal traveled to these countries to brief intelligence agencies on Russian spycraft and the activities of former colleagues.

It said that Britain has suggested that the Kremlin staged its attack to send the message that it would never forgive or forget any traitor. To buttress their case, the British authorities have portrayed Skripal as a symbolic victim who was living quietly in semiretirement in Salisbury after being swapped in a high-profile spy exchange in 2010.

But in the years prior to the poisoning, Skripal, a veteran of Russia's military intelligence agency, the GRU, apparently traveled widely, offering briefings on Russia to foreign intelligence operatives, according to European officials, who only spoke on condition of anonymity. The meetings were almost certainly approved and possibly facilitated by British authorities as a way to both educate their allies and provide Skripal with income, The NYT said.

He met with Czech intelligence officials on several occasions, and visited Estonia in 2016 to meet with local spies, according to the report. Such visits were neither illegal nor unusual for defectors. But they meant that Skripal was meeting with intelligence officers working to thwart Russian operations in Europe, introducing the possibility that his poisoning was a narrower act of retribution.

Intelligence services not keen to comment

There is no way to know for certain whether Skripal's travels made him a target, or even whether the Russian government knew about them, the paper said. The trips were kept secret, known only to a select few intelligence agents. Not a single official from the spy services in the Czech Republic or Estonia would discuss the details publicly.

Asked whether Skripal had met in recent years with intelligence agents in Spain, where he had once worked as a double agent, a spokesman for the country's foreign intelligence service, the CNI, responded that the question "is a red line we cannot cross."

Skripal arrived in Prague in 2012 shortly after his wife, Lyudmila, succumbed to uterine cancer. He was grieving, but nevertheless in good spirits when he met with officers from at least one of the Czech Republic's three intelligence services, according to a Czech official with knowledge of the meetings. Some details of the visit were first reported over the weekend by the Czech weekly Respekt, and were confirmed independently by The NYT.

During the brief visit, Skripal provided Czech intelligence with information about GRU officers operating in Europe. His information was dated; he retired from the GRU in 1999. Even so, the Czech officers found his knowledge to be valuable. Many GRU agents he worked with in the 1990s were still active, one official said.

Skripal was so helpful that Czech intelligence officers continued to meet with him, the official said, making several trips to the U.K. in subsequent years, though the exact dates are unclear.

Officials were more circumspect about Skripal's visit to Estonia, with one describing it as "very sensitive information." A senior European official with knowledge of the trip confirmed that the former Russian agent met secretly with a select group of intelligence officers in June 2016, though it is not clear what they discussed. British intelligence services helped facilitate the meeting, the official said.

A spokesperson for the British Home Office also declined to comment.

Skripals poisoned with novichok

On March 4, Skripal and his daughter Yulia were found semiconscious on a park bench in the Southern England town of Salisbury. Officials later determined that they had been poisoned with novichok, a deadly nerve agent developed in the Soviet Union. The poisoning, in which Russia has denied any involvement, touched off a furious confrontation between Russia and the West that has led to the expulsion of more than 150 Russian diplomats from more than two dozen countries.

In 2006, a Russian military court convicted Skripal of selling out fellow Russian spies in exchange for payments from British agents. He was serving a 13-year sentence when he was unexpectedly sent to the U.K. in the 2010 spy swap.