The 34-year-old officer, sometimes crying, testified Friday at Sinclair's court-martial at Fort Bragg in North Carolina, the New York Times reported. She said Sinclair's behavior became increasingly violent and that he sometimes forced her into sex.
The woman said Sinclair became angry when she said she wanted to meet his wife. "He told me that if I ever said anything to her or anyone else about he and I, he would kill me," she testified.
Sinclair has pleaded guilty to some of the charges against him, admitting having sexual relations with the captain and two other women and to possession of pornography. He denies sexual assault.
Defense lawyers say the captain was angry at Sinclair because he would not leave his wife for her.
"She spent three years in this relationship, and she thinks she wasted her time, and she's furious about it," lawyer Ellen Brotman said.
The defense is scheduled to begin cross-examination Monday.