WASHINGTON—The special counsel Robert Mueller announced on Saturday that he was closing the Justice Department’s Russia investigation, “effective immediately,” after hearing that President Vladimir Putin, of Russia, said he was innocent of any election meddling.

Moments after learning about Putin’s assertion, Mueller hastily assembled his staff of investigators to inform them that, now that Putin had fully exonerated himself, there was no point in continuing the probe.

“Vladimir Putin says he did nothing,” Mueller told his staff. “That’s good enough for me.”

Speaking later to reporters, Mueller said that, by disbanding his investigation, he was following the time-honored law-enforcement tradition of taking a suspect’s word for it.

“For the past several months, we’ve assembled tax records, cell-phone recordings, bank transfers, and e-mail communications that indicated Russia was involved in the election,” a visibly shaken Mueller said. “Somehow, we got it wrong—very, very wrong.”

As for his future plans, Mueller said that his first order of business was to write a hearfelt letter of apology to Putin.

“I feel terrible knowing that I’ve spent all this time investigating a person who did absolutely nothing wrong,” he said. “Hindsight is 20/20, but I really should have called him first and asked him what really happened.”