The leader of one of Congress' four probes, Sen. Lindsey Graham, said he was convinced that Mueller's appointment would hinder Congress' probing.
"In terms of Congress's role and the public's access to what happened, there's going to be a diminishment here," Graham said. "The shape and scope of the congressional investigation has dramatically changed. I respect the decision."
Senate intelligence Chairman Richard Burr, a North Carolina Republican, noted that Senate investigators were still pursuing former national security adviser Michael Flynn, even though they do not yet know how Flynn will respond to their subpoena for documents.
"We're continuing on with a lot of interviews and through those interviews it leads us to additional document requests and additional individuals we¹d like to talk to," Burr said.
The ranking Democrat on the committee, Sen. Mark Warner, a Virginia Democrat running the Senate investigation said of the special counsel news: "I think it means our investigation is more important than ever."
"The chairman and I have talked about doubling down, that¹s the message we've conveyed to the staff," Warner said. "We still would like to hear from (former FBI Director James) Comey, we still believe that his public appearance in an open session, and if needed, a closed session, is the right next step for him. The American people need to hear his side of the story, so we're at it."
Lawmakers were still picking through the fallout of Wednesday's stunning announcement by deputy Attorney General Rod Rosenstein that he appointed Mueller to direct the special investigation into ties between Trump aides and Russia. Democrats and Republicans both looked relieved -- one House Democrat even said he had the best sleep Wednesday night after months of waking up in the middle of the night over concerns about Russia's influence.
House investigators said they were plowing full speed ahead with their own inquiry -- one source noted that the appointment of a special counsel didn't matter much because Comey had not been providing House investigators much access to begin with.
The leaders of the House Russia investigation, Rep. Mike Conaway and Rep. Adam Schiff, announced Thursday that they sent a formal request to the FBI seeking records of Comey's discussions with Trump.