Donald Trump's son-in-law Jared Kushner is the "very senior transition official" referred to in court papers filed in the case of Michael Flynn who has pleaded guilty to lying to the FBI, sources said.
According to court papers filed on Friday, the "very senior transition official" directed former national security adviser Flynn to contact foreign governments, including Russia, about a UN resolution regarding Israeli settlements.
Flynn agreed to co-operate with special counsel Robert Mueller's probe, which focuses on Russian meddling in the 2016 election and possible co-ordination between Moscow and Mr Trump's campaign aimed at sending the Republican businessman to the White House
He told investigators a senior member of the president's transition team directed him to make contact with Russian officials in December 2016.
Flynn was an early and vocal Trump supporter on the campaign trail and was present for big moments in the campaign, the following transition period and the early days of Mr Trump's presidency, making him a valuable potential tool for prosecutors and agents.
His business dealings and foreign interactions have made him a central focus of Mr Mueller's investigation.
Flynn admitted to lying about his conversations with Russia's ambassador to the US during the transition period before Mr Trump's inauguration.
In a statement, the retired Army lieutenant general accepted responsibility for his actions and added: "My guilty plea and agreement to co-operate with the Special Counsel's Office reflect a decision I made in the best interests of my family and of our country."
Former deputy national security adviser KT McFarland is another unnamed senior official referred to in the court papers filed in the case, sources said.
She was involved in a discussion with Flynn about what he would say to Russian government officials in response to US sanctions imposed on Russia last year. That is according to two former officials on Mr Trump's transition team who spoke on condition of anonymity.
Papers filed by Mr Mueller show Flynn contacted McFarland on December 29 2016 to discuss what, if anything, he would communicate to the Russians about the sanctions. Flynn admitted on Friday to lying to the FBI about the contents of that conversation.
McFarland is awaiting Senate confirmation as ambassador to Singapore.