Sean Spicer takes notes as President Donald Trump and NATO Secretary-General Jens Stoltenberg hold a news conference at the White House in April. Associated Press/Andrew Harnik

Sean Spicer was reportedly known for being a copious note-taker during his time working on Donald Trump's campaign and later as the White House press secretary — a habit that could prove relevant as the FBI's Russia investigation heats up.

Spicer filled "notebook after notebook," to the extent that his subordinates joked that he would eventually write a tell-all memoir, the news website Axios reported Thursday, citing Spicer's former colleagues.

"Sean documented everything," one source familiar with Spicer's note-taking told Axios.

Spicer's name has already been floated as a possible witness whom Robert Mueller, the special counsel leading the investigation, may pursue. The Washington Post reported earlier this month that Mueller had alerted the White House that he would most likely seek to interview Spicer, along with five other current and former associates of Trump's.

According to The Post, the associates are thought to have been present during internal discussions of interest to the FBI's investigation into Russia's interference in the 2016 presidential election, which includes whether the Trump campaign colluded with Moscow.

Thorough note-taking is a departure from typical White House practice — officials from previous administrations told Axios that they deliberately took scant notes during their time in the West Wing because of past investigations.

Another White House official told Axios that "people are going to wish they'd been nicer to Sean," adding, "He was in a lot of meetings."

When asked by Mike Allen, Axios' executive editor, about his notes, Spicer reportedly became irate and accused Allen of harassment.

"Please refrain from sending me unsolicited texts and emails," Spicer told Allen in an email, according to Axios' report. "Should you not do so I will contact the appropriate legal authorities to address your harassment."