A man who allegedly battered a trans woman to death when he discovered she had been born physically male has gone on trial in Colorado, US.

Prosecutors say Allen Andrade, 32, beat 18-year-old Angie Zapata to death last July with a fire extinguisher when he found out about her past. The pair had met on a dating website and spent the night together.

Her body was found hidden under a blanket at her apartment in Greeley by her sister.

Ms Zapata began living as a woman when she was 16 and had wanted to dress in a feminine style since the age of seven, her family said.

Her sister Monica Zapata told AP how she attracted a number of men but faced sadness when each relationship ended.

“I heard her conversations. ‘Hey, before we go any further, I have something to tell you.’ A lot of times I would see she would be on the phone and she’d be, ‘Hello? Hello?” Ms Zapata said. “Some would hang up on her. Other times, some would be asking questions.”

“They’d say, ‘I’d thought I could, but I can’t,’” she recalled. “She would cry.”

According to the New York and Los Angeles-based Gay & Lesbian Alliance Against Defamation, Andrade is believed to be the first person tried for a hate crime under the sexual orientation section of Colorado’s hate crime law.

He told police he had hit Ms Zapata twice in the head with the extinguisher and thought he had “killed it” before hitting her again as she tried to stand up.

Last month, a judge disallowed part of Andrade’s confession, saying police had not honoured his request to remain silent during part of his interview.

Evidence including tape-recorded prison calls in which Andrade allegedly told his girlfriend that he “snapped” and that “gay things need to die” will be permitted during the trial.