From CNN:

Since the November election, US intelligence and law enforcement agencies have detected an increase in suspected Russian intelligence officers entering the US under the guise of other business, according to multiple current and former senior US intelligence officials. The Russians are believed to now have nearly 150 suspected intelligence operatives in the US, these sources said. Officials who spoke to CNN say the Russians are replenishing their ranks after the US in December expelled 35 Russian diplomats suspected of spying in retaliation for election-meddling.

"The concerning point with Russia is the volume of people that are coming to the US. They have a lot more intelligence officers in the US" compared to what they have in other countries, one of the former intelligence officials says.

...

Fueling law enforcement officials' concern is that the Russians are targeting people in the US who can provide access to classified information, in addition to ongoing efforts to hack the US government for intelligence, according to several of the officials. In some cases, Russian spies have tried to gain employment at places with sensitive information as part of their intelligence-gathering efforts, the sources say.

But that hasn't stopped the State Department from issuing the temporary duty visas — also known as TDY — to the suspected Russian intelligence officers. US intelligence officials who spoke to CNN expressed concerns about the number of temporary visas the State Department has issued to Russian travelers...