WASHINGTON — President Trump was informed more than two weeks ago that his national security adviser, Michael T. Flynn, had not told the truth about his interactions with Russia’s ambassador, and the president eventually asked for Mr. Flynn’s resignation after concluding he could not be trusted, a White House official said on Tuesday.

Sean Spicer, the White House press secretary, said the president’s team had been “reviewing and evaluating this issue with respect to General Flynn on a daily basis for a few weeks trying to ascertain the truth.” While Mr. Trump and his advisers ultimately concluded that there was no violation of law, the president decided Mr. Flynn could no longer serve in his position.

“The evolving and eroding level of trust as a result of this situation and a series of other questionable instances is what led the president to ask for General Flynn’s resignation,” Mr. Spicer said at his daily briefing. He did not elaborate on the “other questionable instances.”

As leaders of both parties indicated that Congress would investigate, Mr. Spicer’s comments were the first public confirmation by the White House that Mr. Trump was aware of the allegations against Mr. Flynn before they were reported last week. They also contradicted the previous description of the national security adviser’s abrupt departure on Monday night, attributing the decision to Mr. Trump rather than Mr. Flynn.