Washington(CNN) Among the dozens of Russian diplomats the US expelled last month were suspected spies who US law enforcement and intelligence officials believe were tracking Russian defectors and their families who had resettled in the US, officials briefed on the matter tell CNN.

In at least one instance, suspected Russian spies were believed to be casing someone who was part of a CIA program that provided new identities to protect resettled Russians, the officials said.

That episode and other US intelligence raised concerns that the Russians were preparing to target Russian émigrés in the US labeled by the Kremlin as traitors or enemies, law enforcement and intelligence officials said.

The CIA declined to comment. The White House declined to comment.

The Russian embassy did not respond to a request for comment.

The concerns have been heightened following the poisoning in the UK of a former Russian spy, Sergei Skripal, and his daughter, who is a Russian citizen. British and US officials have blamed Russian intelligence for the use of a nerve agent in the attempted poisoning.

The White House last month said it was expelling 60 Russians as a measure of solidarity with the UK over the Skripal poisoning case.

Officials in both the US and UK have warned that the Russian government appears emboldened to carry out assassinations in western Democracies.

Democrats on the Senate Foreign Relations Committee produced a report earlier this year raised the issue of the suspicious deaths of more than two dozen critics of Russian President Vladimir Putin during his time in power. The Russian security services are suspected in many of the deaths, the report said, noting a Russian law passed "in July 2006 that permits the assassination of 'enemies of the Russian regime' who live abroad."

"The trail of mysterious deaths, all of which happened to people who possessed information that the Kremlin did not want made public, should not be ignored by Western countries on the assumption that they are safe from these extreme measures," said the Senate Democrats in their report.

Clarification: This story has been updated to clarify Yulia Skripal's citizenship and to reflect that Skripal and her father were poisoned.