The investigation into Russia’s election meddling and any potential ties between Trump campaign staff members and the Kremlin is reportedly focusing on an 18-day period in early 2017 leading up to former White House national security adviser Michael Flynn’s resignation.

According to NBC News, the investigation is honing in on the time period between Jan. 26 and Feb. 13, which was the day Flynn resigned.

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On Jan 26, former acting Attorney General Sally Yates told White House Counsel Don McGahn that Flynn was not telling White House officials the truth about his conversation with the Russian ambassador to the U.S., Sergey Kislyak.

Flynn at the time said he “inadvertently” provided Vice President Pence with “incomplete information” regarding his conversations with Kislyak.

Special counsel Robert Mueller is investigating why it took Flynn 18 days to resign after the president was briefed on Yates’s caution to McGahn, the report said.

Two individuals confirmed to NBC News that the investigation appears to be focused on President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE potentially obstructing justice during that timeline.

Sources told NBC News that it looks like Mueller wants to know if Trump instructed Flynn to lie to officials.

Flynn earlier this month pleaded guilty to lying to the FBI, becoming the first former Trump administration official charged in Mueller’s probe.