With James Comey out at the FBI, Acting Director Andrew McCabe has taken over the investigation of Russian interference in the 2016 election.
Sources tell CBS News the FBI investigation had been picking up speed in recent weeks, and that it now includes grand jury subpoenas.
Comey's request last week for more money and resources was a sign the probe was far from over. Now, sources say there is a worry the investigation, as well as separate congressional probes, will slow down.
"An interruption in, in any of the access we have to documents and to personnel would be harmful to our investigation," said Republican Richard Burr, chairman of the Senate Intelligence Committee. "It wouldn't in any way, shape or form preclude us from coming to a conclusion. But it might delay us."
In recent weeks, sources say FBI agents in this country and overseas have been intensely focused on following the money -- trying to identify who might have paid the hackers involved in Russia's campaign to interfere in the election.
They're also scrutinizing the activities of President Trump's associates, including former National Security Adviser Michael Flynn, former Campaign Chairman Paul Manafort and former campaign foreign policy adviser Carter Page. All have ties to or contacts with Russian business or government officials.
Former FBI Assistant Director Ron Hosko worries that Comey's firing could have a chilling effect.
"If the timing of this termination lines up with timing about a slowdown in the investigation, the Russia investigation, then I think there are serious, serious questions that are going to need to be answered by this White House and by the Department of Justice," Hosko said.
The White House says McCabe met with Mr. Trump Wednesday because the president wanted to discuss bureau morale.
On Thursday, McCabe will be testifying on Capitol Hill at a previously scheduled hearing.
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