Lt. Abel Herrera said the 17-year-old girl, who is eight months pregnant, told human rights officials in Esteli that she shot her live-in boyfriend, Kenneth A. Kinzel, on May 12 because he threatened to kill her and beat his unborn fetus out of her.
The girl said her 14-year-old sister and 19-year-old brother, who has Down's Syndrome, helped her cut up the body to put it in plastic bags, and then they drove outside the city and buried the bags in two places in northern Nicaragua, Herrera said.
Police located the remains and were conducting DNA tests to confirm their identity.
The teenager and her American lover lived together in Esteli, Herrera said.
The U.S. Embassy in Managua confirmed the man's death and his name, but did not immediately have information about his hometown.
Also on Tuesday, the embassy confirmed that an American woman, Lemon E. Groves, 49, had died of injuries suffered when she was attacked in her home in the Nicaraguan city of Grenada last week.
Embassy officials had no further information on that death.