Kevin Rideout, an American missionary pilot who was seized in Niger last fall, is in U.S. custody and on his way to the United States, two U.S. officials said on Thursday.

The release of Mr. Rideout, a 50-year-old pilot who was kidnapped in October outside his home in Niger’s capital, Niamey, while working with an evangelical group, has been a priority for the White House and the military’s Africa Command since his abduction.

Details of how and precisely where Mr. Rideout was released were unclear late Thursday, said the two officials, who spoke on the condition of anonymity to discuss operational matters. It was also unclear which terrorist or criminal group held Mr. Rideout at the end of his captivity, as he may have been moved recently, one of the two officials said.

Mr. Rideout was in F.B.I. custody and in good health based on proof-of-life images that were provided to American officials in the past 48 hours, one of the officials said.

The White House and Pentagon did not respond immediately to requests for comment late Thursday.

Mr. Rideout had been working in Niger as a Christian missionary for 19 years. He was also a pilot for Serving in Mission, an organization based in Charlotte, N.C., that describes itself as a “global mission family serving in 70 countries” with a focus on outreach to communities with few Christians.

In the past year, the U.S. military has been helping neighboring Nigeria with training and surveillance in its fight against jihadists. Nigeria also allowed the United States to fly surveillance missions from its bases, in part to try to locate Mr. Rideout.

In December, the United States carried out missile strikes in northwestern Nigeria that President Trump said targeted Islamic State fighters. That operation was in coordination with the Nigerian military, the Pentagon said at the time.

Thousands of Christians and Muslims have been killed in Nigeria in land disputes, sectarian violence and terrorism, which Christian activists and Republican lawmakers in the United States have viewed as the persecution of Christians. There is no clear evidence to show that Christians are attacked more frequently than any other religious group in Nigeria, analysts say.

In May, U.S. and Nigerian forces killed Abu Bilal al-Minuki, one of the Islamic State’s highest-ranking leaders, in an operation in Nigeria.

American officials said that Mr. al-Minuki oversaw ISIS’s global operations, media, recruitment and finances. In that position, the officials said, Mr. al-Minuki was directly involved in plots like the kidnapping of Mr. Rideout.