WASHINGTON — President Trump cleared up one of the capital’s least suspenseful mysteries on Thursday, acknowledging that he did not record conversations with James B. Comey, the F.B.I. director he fired in anger over an investigation into his campaign’s possible ties to Russia.

Meeting a self-imposed deadline of this week to resolve questions he himself raised by implying that he had taped Mr. Comey, Mr. Trump said on Twitter that he had not made tapes of what Mr. Comey has testified were attempts by the president to derail the Justice Department’s investigation.

But if few people believed that Mr. Trump actually possessed recordings, his motives in warning Mr. Comey that he might have taped him remain a mystery, particularly since it set off a chain of events that accelerated, rather than slowed, the investigation into Mr. Trump and Russia.

Mr. Comey testified that it was Mr. Trump’s veiled threat of tapes that led him to authorize the disclosure of memos of his conversations with the president — the details of which prompted the Justice Department to appoint a special counsel to look into the case.