Special counsel Robert Mueller is trying to piece together what transpired inside the White House over a critical 18-day period that began when senior officials were told that national security adviser Michael Flynn was susceptible to blackmail by Russia, multiple people familiar with the matter told NBC News.

The questions about what happened between Jan. 26 and Flynn's firing on Feb. 13 appear to relate to possible obstruction of justice by President Donald Trump, say two people familiar with Mueller's investigation into Russia's election meddling and potential collusion with the Trump campaign.

Multiple sources say that during interviews, Mueller's investigators have asked witnesses, including White House counsel Don McGahn and others who have worked in the West Wing, to go through each day that Flynn remained as national security adviser and describe in detail what they knew was happening inside the White House as it related to Flynn.

Some of those interviewed by Mueller's team believe the goal is in part to determine if there was a deliberate effort by Trump or top officials in the West Wing to cover up the information about Flynn that Sally Yates, then the acting attorney general, conveyed to McGahn on Jan. 26. In addition to Flynn, McGahn is also expected to be critical to federal investigators trying to piece together a timeline of those 18 days.

Neither McGahn's lawyer nor the White House responded to requests for comment. A spokesman for the Special Counsel's office declined to comment.

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