Special counsel Robert Mueller's investigators are asking White House officials to walk them through 18 days early in the Trump presidency in a sign that they are probing possible obstruction of justice by the president, according to NBC News.
The period began Jan. 26, when White House counsel Don McGahn was told National Security Adviser Michael Flynn could be blackmailed by Moscow for lying about his contacts with Russia, and ended Feb. 13, the day President Donald Trump fired Flynn, NBC reported Monday.
The network cited multiple people familiar with the matter.
Flynn and McGahn are expected to offer insight into the approximately three-week period, NBC said. Flynn pleaded guilty Dec. 1 to lying to the FBI and has been cooperating with the special counsel's office.
Just four days after Flynn was sworn in as national security advisor, then-acting attorney general Sally Yates warned McGahn that Flynn was in danger of becoming the target of Russian blackmail. She said he was susceptible because of false statements he had made about his contacts with the Russian ambassador to Vice President Mike Pence, according to Yates' May congressional testimony.
Yet for 18 days after that Jan. 26 meeting, Flynn continued to serve as the president's top advisor on national security, surprising Justice Department officials who had expected Flynn to be fired immediately, NBC News reported.
Meanwhile, Trump fired Yates Jan. 30 after she refused to defend an executive order that limited travel to the United States for people from predominantly Muslim countries.
Sources cited by NBC News said they believe Mueller's goal is to determine whether Trump or his top advisors sought to cover up the information about Flynn that Yates had warned about.
NBC said Mueller's team also "appears to be interested" in whether Trump directed Flynn to lie to the FBI.
It is unclear when the president knew that Flynn lied to the FBI. Yates said she declined to tell McGahn at the Jan. 26 meeting whether Flynn had done so because it would have been inappropriate to discuss.
A Dec. 2 tweet sent from the president's account suggested that the president knew Flynn had lied to the FBI when Flynn was fired. After the wording of the tweet drew scrutiny, presidential attorney John Dowd claimed authorship of the tweet, and said the implication that Trump knew Flynn lied to the FBI was a mistake.
Interactions between the president and former FBI director James Comey have also raised the specter of obstruction of justice.
On Jan. 27, the day after the McGahn-Yates meeting, Trump invited Comey to a private dinner and told him, "I need loyalty, I expect loyalty," the fired FBI director told congressional investigators.
Comey also testified that the president later asked him to see to "letting Flynn go."
Trump fired Comey in May, at first citing his handling of the Hillary Clinton email investigation and later offering a variety of reasons for the dismissal, including the Russia probe.
The president has denied asking Comey to end the investigation into Flynn, and Dowd has argued that a president "cannot obstruct justice."
A spokesperson for the special counsel's office declined to comment to CNBC.
Attorneys for the president and McGahn did not respond to CNBC's request for comment.
WATCH: Deutsche Bank reportedly receives subpoena from Mueller on Trump accounts
