U.S. President Donald Trump takes the stage for a rally at the U.S. Cellular Center in Cedar Rapids, Iowa, U.S. Thomson Reuters President Donald Trump could have landed himself in more trouble by admitting that he doesn't have tapes of his conversations with former FBI Director James Comey — and saying he floated the suggestion he did in an attempt to influence Comey's congressional testimony.

It all started when Trump tweeted: "James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press!"

The tweet came three days after Trump stunningly fired Comey. The president and his former FBI director had been at odds amid an investigation into whether anyone associated with Trump's campaign colluded with Russian officials to meddle in the 2016 election.

Trump admitted this week that he didn't record his conversations with Comey. He suggested in a Friday interview that he floated the idea of tapes to pressure Comey to keep his story straight when he appeared to testify before a Senate committee about the Russia probe.

"When he found out that I, you know, that there may be tapes out there, whether it's governmental tapes or anything else — and who knows — I think his story may have changed," Trump told "Fox & Friends."

James Robenalt, a partner at Thompson Hine law firm who wrote a book about the Watergate scandal, said Trump may have complicated things for himself with this statement. The president is now facing an investigation into whether he obstructed justice with his firing of Comey.

"He's trying to influence a witness," Robenalt said. "There are multiple obstruction crimes under our law, and one of them is intimidating a witness."

The problem is that Trump "seems to be conceding that he was trying to influence — if not intimidate — Comey and his testimony," Robenalt added.

"The fact that [Trump] knew when he said it that he didn't have tapes … to me is an indicator that he was doing it for an improper purpose," he said.

Robenalt said investigators would likely try to establish a pattern of obstruction, and this could contribute to that overall picture.

Trump could claim that he wasn't trying to pressure Comey to lie, but rather that he was pressuring Comey to tell the truth with his implication that there were recordings of their conversations.

And here, there are multiple events that led to the obstruction of justice investigation — Trump firing Comey, and what Comey has characterized as a request to lay off of his investigation into former national security adviser Michael Flynn. The possible intimidation could be viewed as another factor.

"That's a pattern of conduct that fits into an obstruction mode where they're trying to act in a way that obstructs an investigation," Robenalt said.

Robenalt also drew parallels between Trump and former President Richard Nixon, who resigned in the wake of the Watergate scandal after recordings emerged of his conversations from inside the White House. After news of the tapes emerged, the White House admitted that a crucial 18.5 minute conversation between Nixon and his chief of staff after the Watergate break-in had been erased.

"In the middle of the question of who erased the tape," another Nixon chief of staff, Alexander Haig, "said there may have been sinister forces involved, which is a direct echo of Trump," Robenalt said.

In explaining his tweet, Trump referenced alleged government surveillance that he believes is being conducted, saying, "you never know what's happening when you see that the Obama administration, and perhaps longer than that, was doing all of unmasking and surveillance."

"This whole mention of sinister forces was like a way of diverting attention in some strange way from what really happened," Robenalt said. "With Trump saying maybe somebody else did taping, it makes me wonder are there tapes, did tapes exist at one point that have now been destroyed, are there tapes out there that he's aware of but that he didn't do himself."