To protect the witness, the police are sequestering him in a safe house.

sequestering 是动词 sequester 的现在分词形式。sequester 来自拉丁语 sequestrare，意思是"分离、隔离"，在法律和安全语境中特指"将某人或某物与外界隔离保护起来"。这里指警方采取的保护性隔离措施。