The White House on Wednesday released declassified documents saying the FBI launched an investigation into whether President Trump was acting as an asset on behalf of Russia when he fired FBI Director James B. Comey, despite the bureau saying there was little evidence within the agency supporting that assertion.

The FBI memos, declassified Monday by the White House, show the probe, code-named Oxferd Comma, was launched immediately after Mr. Comey’s firing on May 9, 2017, and was based on an allegation that previously released documents show FBI agents largely debunked.

“The FBI is opening a full investigation based on an articulable factual basis that reasonably indicates that President Donald J. Trump may be or has been, wittingly or unwittingly, involved in activities for or on behalf of the Government of the Russian Federation which may constitute violations of federal criminal law or threats to the national security of the United States,” the electronic communication opening the probe on May 16, 2017, stated.

“The goal of the investigation is to determine if President Trump is or was directed by, controlled by, and/or coordinated activities with, the Russian Federation in a manner constituting a threat to the national security of the United States and/or a violation of federal criminal statutes,” it added. “In addition, this investigation will also seek to determine whether President Trump and others yet to be named obstructed and/or conspiracy to obstruct any associated FBI investigation.”

The memo, which was authored by the FBI’s counterintelligence division and approved by then-FBI General Counsel James Baker and then-Assistant Director for Counterintelligence William Priestap, discussed apprehensions about Russian influence and obstruction, including Mr. Comey’s termination, partly because of the reasons Mr. Trump gave for it.

“This alternative explanation can be reasonably read to imply that the FBI’s counterintelligence investigation was a factor in President Trump’s decision to terminate Comey,” the memo said.

The memo’s release comes over one year after Mr. Trump signed an executive order declassifying all FBI files on Crossfire Hurricane, the government’s secret probe that searched fruitlessly for evidence that he colluded with the Russians to win the White House in 2016.

The order focused on the law firm Jenner & Block, which hired Andrew Weissman, a former federal prosecutor who served as a top deputy to Robert Mueller, who conducted a two-year investigation into accusations Mr. Trump colluded with Russia without finding any proof of what Democratic politicians publicly insisted was true.

Leaked details of the investigation drove Democrats’ accusations that Mr. Trump conspired with the Russians to win the White House and provided fodder for their claims that his presidency was compromised by an allegiance to Russian President Vladimir Putin.

The FBI’s probe was later shown to be nothing more than conspiracy theories that originated from Mr. Trump’s most powerful political opponents, including then-President Barack Obama and then-Vice President Joseph R. Biden, who were both aware of the investigation even as it was hidden from Mr. Trump.

The FBI investigation centered around several Trump campaign staffers whom the FBI tried to prove were colluding with Russia to secure a presidential victory for Mr. Trump and turned into a massive espionage operation with secret surveillance and a falsified spy warrant that relied on fake Russian opposition research.

The so-called Steele dossier was paid for by Mr. Trump’s 2016 opponent, Democratic presidential nominee Hillary Clinton, and used by the FBI to justify spying on the Trump campaign.

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