The special counsel Robert Mueller is reportedly more interested in the activities of Donald Trump during his time as president than during his campaign activities.

It suggests he's building a case that Trump obstructed justice by firing FBI Director James Comey.

Trump went on a Twitter tirade against the FBI over the weekend after reports that Mueller had subpoenaed the Trump Organization for documents related to its efforts to build a Trump Tower in Moscow.

Comey and Andrew McCabe, another recently fired top FBI official, kept detailed memos about their conversations with Trump and will most likely be critical witnesses in the obstruction investigation.

The special counsel Robert Mueller is reportedly more interested in the activities of Donald Trump during his presidency than during the campaign, and it could reveal Mueller's endgame as the Russia investigation stretches into its second year.

Mueller has specifically been focusing on Trump's firing of James Comey as FBI director and of Michael Flynn as national security adviser, Mike Allen, the cofounder of the news website Axios, told MSNBC's "First Look" on Monday.

Axios said that focus gave a "huge clue" as to Mueller's endgame in the investigations.

"That suggests he's looking more at obstruction of justice, things that might have happened in office, than he is at collusion, maybe something that happened during the campaign," Allen said. "Both of those stories have a real Russia thread through them."

Allen's reporting follows a heavy effort from Trump to discredit Mueller's investigation with Twitter swipes at the FBI and amid the firing of the FBI's deputy director, Andrew McCabe, just hours before his full pension took effect.

Trump has repeatedly denied any coordination with Russia leading up to the 2016 election, something he brought up in his recent Twitter tirade, but the investigation into whether he obstructed justice during the Russia investigation is a separate legal matter.

Trump's legal team has been working for months to sidestep or significantly narrow the scope of an interview between Mueller and their client, and on Saturday morning Trump's personal defense attorney John Dowd told The Daily Beast the Russia investigation should be shut down.

The New York Times reported on Thursday that Mueller had subpoenaed the Trump Organization for documents related to its efforts to build a Trump Tower in Moscow, something that reportedly angered Trump just before his Twitter attacks on the FBI's investigation.

Comey and McCabe kept detailed memos about their conversations with Trump and will most likely be critical witnesses in the obstruction investigation.

But Trump preemptively attacked McCabe's memos, suggesting they were politically motivated and should be called "Fake Memos."

Sonam Sheth contributed to this report.