Up to one million people, or about 7% of the Muslim population in China’s Xinjiang region, have now been incarcerated in an expanding network of “political re-education” camps, according to U.S. officials and United Nations experts.

As the camps have swelled in size, some Uighurs living outside China say that relatives—mainly, but not all, older people—have died in detention or shortly after their release.

Satellite images reviewed by The Wall Street Journal and a specialist in photo analysis show that camps have been growing. Construction work has been carried out on some within the past two weeks, including at one near the western city of Kashgar that has doubled in size since Journal reporters visited in November.