(NEW YORK) — The former girlfriend of accused killer Brendt Christensen, who recorded his alleged admission of brutally killing 26-year-old Chinese scholar Yingying Zhang, testified Wednesday that Christensen seemed “proud” and “excited” when describing how he killed her.On June 16, 2017, Terra Bullis, who said she started dating Christensen in April that year after meeting on OkCupid, agreed to use a hidden audio device for the FBI.”I was emotionally attached to this person and wanted to know if they had done anything or not,” Bullis, 32, said she told prosecutors after being asked why she agreed to wear a wire. “I would be able to inform both myself and potentially law enforcement.”She said on the stand that she recorded Christensen, 29, a total of nine times and that his alleged admission was recorded on June 29, 2017, as he and Bullis attended a vigil for Zhang.The defense did not dispute that Christensen’s voice was on the recording.In the June 29 audio, Christensen is heard describing Zhang’s “valiant” efforts to fight back, saying it was “supernatural almost how [Zhang] just didn’t give up.” Bullis said he seemed “boastful” and was bragging.Christensen went on to say in the recording that he decapitated Zhang to ensure she was deceased after assaulting her with a baseball bat.Christensen, who is currently standing trial in federal court, is accused of kidnapping and killing Zhang, an agriculture researcher.She was last seen on video entering a Saturn Astra on June 9, 2017. The FBI became involved after the video surfaced and the case moved from a disappearance to a possible kidnapping.Within days, investigators tracked the Saturn Astra back to Christensen. Zhang’s body has not been found.Although he had pleaded not guilty, Christensen’s attorney in his opening statement admitted that his client killed and kidnapped Zhang.Bullis described the fear she said she felt throughout her undercover work. At one point, prosecutor James Nelson asked about loud thumping that could be heard through much of the June 29 audio.”That was my heartbeat,” Bullis responded.Bullis said Christenesen looked through her phone on June 29, but, she said, she had deleted emails between herself and the FBI moments prior.In that June 29 audio, which was played in court, Christensen is heard telling Bullis several times that Zhang was his 13th victim — adding that he had been killing since he was 19 years old.”Yingying is the only person that produced evidence that leads back to me,” said Christensen in the audio recording. “Number 13.”Bullis testified that Christensen told her he “admired” serial killers like Ted Bundy. He is heard telling Bullis his victim count is “bigger than Jeffrey Dahmer — bigger than John Wayne Gacy,” both notorious American serial killers.She told the prosecutor she agreed to testify in open court “because it was necessary” and that the FBI paid her an approximate seven or eight thousand dollars for her assistance, which she said was to help cover living arrangements after quitting her job shortly after the June 29 recording.The defense cross-examined Bullis for about two hours. Defense attorney Robert Tucker accused Bullis of trying to “implicate” Christensen during their June 29 conversation. “That is not true, sir,” Bullis responded.Tucker pointed out instances in the audio when Bullis seemed to go along with Christensen, stating that she was “attracted” to this serial killer. Bullis responded that she said those words because she was “terrified” and wanted him to continue the conversation.Tucker also noted that Christensen had been drinking at the time of this conversation. Bullis agreed, but said Christensen was not drunk at the time of his statements on tape.In the last clip heard from the June 29 audio, Christensen tells Bullis he would not tell her nor anyone else what he did with Zhang’s body.”No one will ever know where she is,” he is heard saying on the recording. “She’s gone forever.”The defense is looking to spare Christensen from a possible death sentence and added in opening statements that it took issues with the way the prosecution “says the events [of the crime] occurred.”Copyright © 2019, ABC Radio. All rights reserved.