Of course, it's possible to obstruct justice without colluding.
Earlier, Mr Trump tweeted that a new report suggesting that special counsel Robert Mueller may investigate him for possible obstruction of justice after he sacked Federal Bureau of Investigation director James Comey is a "phony story".
Donald Trump has told his Twitter followers they are witnessing the "single greatest witch hunt in American history".
The investigation, the president added is being led by "very bad and conflicted people". He said he didn't know if Trump obstructed justice, but said it was for Mueller to decide.
A source familiar with the Mueller investigation confirmed the report, saying an examination of possible obstruction of justice charges was "unavoidable" given testimony by former FBI Director James Comey, although the issue may not become the main focus of the probe.
But, "Officials say that changed shortly after Comey's firing", the Post reports.
It stands to reason that the circumstances surrounding Comey's firing would now be at the center of Mueller's query.
The Post quoted five people briefed on the requests, saying those who agreed to be interviewed include national intelligence director Daniel Coats, Admiral Mike Rogers who heads the NSA, and his recently departed deputy, Richard Ledgett.
A spokesman for the office of the special counsel declined to comment, and so did a representative for the director of National Intelligence.
The special counsel is following two major lines of investigation, said one USA official familiar with the rough outlines of Mueller's probe who spoke on condition of anonymity.
Trump's young presidency has been battered by allegations - under investigation both by Congress and the FBI - that Russian Federation interfered to sway the 2016 election in his favour, in possible collusion with Trump's campaign team. He's also hired Andrew Weissmann, who led the Enron investigation.
Comey had accused Trump of firing him to try to undermine the FBI's investigation of a possible collusion between the Trump Campaign and Russian Federation in alleged efforts to influence last year's presidential election. But then the president himself said he had made up his mind prior to receiving the recommendations from the two top lawyers at the Department of Justice.
Comey testified that initial explanations that he was sacked because of poor leadership were "lies, plain and simple".
"We may have our differences, but we do well, in times like these, to remember that everyone who serves in our nation's capital is here because, above all, they love our country", Trump said.
The panel announced Thursday that Johnson will testify June 21.
Yet Trump's angry tweets on Friday underscored the near-impossible challenge his advisers and legal team have in trying to get him to avoid weighing in on an active probe. "That's Bob Mueller's job to sort that out". After Comey's much-watched Senate testimony, the president said in a press conference that he would testify under oath regarding his interactions and conversations with the former Federal Bureau of Investigation director.
"I think, frankly, our story shows that the president is by no means out of the woods as far as the investigation goes", the Post's Barrett told NPR.
Russian Federation has denied interfering in last year's presidential elections, as has the White House.
While a sitting president is unlikely to face criminal prosecution, obstruction of justice could form the basis for impeachment.
While President Trump denied all the allegations levied on him, Comey revealed that the documented memos were a testament to his claims, a portion of which was published by the The New York Times, with the hope of a new counsel being appointed.