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.