Russia releases a U.S. Navy veteran quietly detained in Kaliningrad in April of last year.

Russia released a Navy veteran who had been detained since April in Kaliningrad, a Russian enclave between Poland and Lithuania, a spokesman for his family announced on Thursday, marking the second time in just over a month that an American has been freed from Russian custody.

Russian officials allowed Taylor Dudley, a 35-year-old U.S. citizen, to cross the Polish border. Mr. Dudley had been held for nine months, the spokesman said, though his case was largely unknown to people outside the U.S. government, his family and advocates.

Mr. Dudley was greeted in Poland by an official from the U.S. Embassy in Warsaw and by Bill Richardson, a former New Mexico congressman and governor who specializes in negotiating the freedom of Americans detained overseas.

Mr. Dudley was backpacking in Europe and had traveled to Poland to attend a music festival, according to the spokesman, Jonathan Franks. He said Mr. Dudley “at some point crossed the Russian border” into Kaliningrad. It remained unclear why Mr. Dudley had crossed into Russian territory and on what charges he might have been held.

There was no evidence that his plight became linked to that of the former U.S. Marine and corporate security consultant Paul Whelan, who was arrested in Moscow in December 2018 and convicted on espionage charges. Mr. Whelan, his family and U.S. officials insist those charges are fabricated.

Still, the release was notable given the extremely fraught relations between Washington and Moscow since Russia’s invasion of Ukraine nearly a year ago, and the extreme difficulty the Biden administration has faced in freeing other Americans held in Russia.

It also came less than five weeks after Russia’s release of the WNBA star Brittney Griner in a prisoner swap that freed the Russian arms dealer Viktor Bout from federal prison. But a U.S. official said that, in this case, the Biden administration had not given Russia anything in exchange for Mr. Dudley’s release.

Ned Price, the State Department spokesman, told reporters at a daily briefing that his ability to comment was limited by a privacy law which an individual must waive to allow the State Department to disclose information about his or her case.

Mr. Richardson said in a statement that he had been working “discreetly” with Mr. Dudley’s family for about six months to bring about his release, along with U.S. State Department officials and Ara Abramyan, a businessman close to President Vladimir V. Putin of Russia, among others.

“It is significant that despite the current environment between our two countries, the Russian authorities did the right thing by releasing Taylor today,” Mr. Richardson said in a statement.

The U.S. official, speaking on the condition of anonymity because of the sensitivity of the matter, credited the work of U.S. officials at embassies in Warsaw and Moscow but said that the administration would not provide further details, out of consideration for the family’s privacy.

U.S. officials had not publicized Mr. Dudley’s case or declared him “wrongfully detained,” as they did Ms. Griner and continue to do for Mr. Whelan.

The circumstances of his travel to Kaliningrad, a vestige of Russian conquest in the Second World War where Moscow maintains a substantial military presence, are murky. Mr. Franks said in his statement that Mr. Dudley had traveled to Europe “to backpack and seek inspiration for a potential book he was working on.”

In September, a television station in Lansing, Mich., reported that a 34-year-old man named Taylor Scott Dudley had gone missing.

“Police say Dudley has health concerns and is in need of medication,” the station, WILX, reported, adding he was “known to walk” in the downtown area. (According to a Facebook page publicizing his disappearance, he was “found safe” days later.)

Meanwhile, Biden officials have been speaking with Russian officials in a continuing effort to secure Mr. Whelan’s freedom, but have made no progress since Ms. Griner’s release. Many analysts believe that Russia would only trade Mr. Whelan for a captured Russian spy, and experts say the U.S. currently does not have any in its custody.

Mr. Whelan’s brother, David, said in an email that before Thursday he had never heard of Mr. Dudley, and added that there are “dozens of US citizens in Russian prisons who have been sentenced and are not designated wrongfully detained by the State Department. There’s no reason to think any of them are connected to Paul’s case.”

Mr. Franks, in his statement, said, “Over the past year, our team traveled to Moscow and the region multiple times, liaising with our Russian counterparts and conduits.”

Mr. Franks and Mr. Richardson also said the effort had been helped by a foundation run by the American entrepreneur Steve Menzies and Vitaly Pruss, a New York-based businessman with ties to Russia.