GREELEY, Colo.  Angie Zapata began living as a woman six years ago even though she was born male and named Justin.

While Ms. Zapata, 18, was accepted by her many friends and five siblings, she was bullied in school and at times was lonely and troubled, an older sister, Monica, said. Eventually, Ms. Zapata dropped out of school and got her own apartment here in Greeley.

It was in that apartment that Ms. Zapata’s badly beaten body was found on July 17.

On Wednesday, the police arrested Allen R. Andrade, 31, and charged him with murder. According to the authorities, Mr. Andrade had gone out on a date with Ms. Zapata, and upon discovering she had male genitalia, beat her to death starting with his fists and then with a fire extinguisher.

Mr. Andrade told investigators that he thought he had “killed it,” according to an affidavit filed by the police. Mr. Andrade, who is in custody, has said nothing publicly about the killing, and his arraignment has not yet been scheduled.