WASHINGTON—After shooting her father and brother in front of her, Boko Haram militants placed Deborah Peter between the corpses, threatening to kill her if she wasn't quiet. It was a day before the Nigerian army came to bring her to a hospital.

Ms. Peter, a native of the Nigerian town where nearly 300 schoolgirls were recently abducted by the militant group, told U.S. House lawmakers on Wednesday that she later learned that Boko Haram later decided it should have killed her. The world "needs to know how horrible"...