Sanders, who is the daughter of former Arkansas Governor Mike Huckabee, led her first White House press briefing last Friday.
Spicer said Tuesday that the bureau chief was sacked following "the recommendation of the attorney general and the deputy attorney general".
Spicer was back to answering questions Friday about Comey's firing.
Indiana Sen. Joe Donnelly said, "I was privileged to serve with (Comey), he's always done the right thing and I know he will here as well". She was calmer and more collected than her boss Sean Spicer, they said, and certainly less condescending. "Our story is consistent", she told reporters.
Tuesday: "It was all him".
A White House official denied that Spicer's job was in jeopardy, saying that using Sanders, 34, to cover Spicer's absence is part of a broader plan to give her some on-camera briefing experience because she is Spicer's chief deputy.More news: Wizards win on Wall's 3-pointer, forcing a Game 7
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White House spokesman Sean Spicer said Tuesday that Rosenstein drafted the memo raising concerns about Comey on his own accord.
Wednesday: "He did have a conversation with the deputy attorney general on Monday where they had come to him to express their concerns". The next day, Sanders said the president asked Rosenstein to put his concerns in writing. He took the recommendation seriously.
"Any investigation that was taking place Monday was taking place today", Sanders said, suggesting that was an indication that Comey's firing would not impact the ongoing probe.
The White House also changed its narrative about whether the president had made a decision to fire Mr Comey based on the recommendations by Mr Rosenstein and Attorney General Jeff Sessions.
Still, a New York Times report yesterday said, "while Mr. Trump has raised the Fox News host Kimberly Guilfoyle to allies as a possible press secretary, he has spent several hours with Mr. Spicer this week, praising his television "ratings" during the briefings". Later, Sanders said, "We want this to come to its conclusion, we want it to come to its conclusion with integrity". "And we think that we've actually, by removing Director Comey, taken steps to make that happen".
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