from the article linked above:
And when a newly inaugurated Mr. Trump sought a loyalty pledge from Mr. Comey and later asked that he end an investigation into the president’s national security adviser, the requests set off discussions among F.B.I. officials about opening an inquiry into whether Mr. Trump had tried to obstruct that case.
But law enforcement officials put off the decision to open the investigation until they had learned more, according to people familiar with their thinking. As for a counterintelligence inquiry, they concluded that they would need strong evidence to take the sensitive step of investigating the president, and they were also concerned that the existence of such an inquiry could be leaked to the news media, undermining the entire investigation into Russia’s meddling in the election.
After Mr. Comey was fired on May 9, 2017, two more of Mr. Trump’s actions prompted them to quickly abandon those reservations.
The first was a letter Mr. Trump wanted to send to Mr. Comey about his firing, but never did, in which he mentioned the Russia investigation. In the letter, Mr. Trump thanked Mr. Comey for previously telling him he was not a subject of the F.B.I.’s Russia investigation.
Mr. Trump’s aides have said that a fuller examination of his comments demonstrates that he did not fire Mr. Comey to end the Russia inquiry. “I might even lengthen out the investigation, but I have to do the right thing for the American people,” Mr. Trump added. “He’s the wrong man for that position.”
F.B.I. officials viewed their decision to move quickly as validated when a comment the president made to visiting Russian officials in the Oval Office shortly after he fired Mr. Comey was revealed days later.
“I just fired the head of the F.B.I. He was crazy, a real nut job,” Mr. Trump said, according to a document summarizing the meeting. “I faced great pressure because of Russia. That’s taken off.”