Not only was he badly injured during the following years, but he was also kept in custody by the Russians for about two years.

custody 是名词，来自拉丁语 custodia（看守、保护），指"拘留、监禁"的状态，在法律语境中表示"被官方拘押"。 在这个句子中，custody 特指"被俄罗斯当局关押"的情况，强调这是一种正式的拘留状态而非普通监禁。