UPDATED: 9/29/2011 – 5:07 p.m.

VALPARAISO, Ind. (CBS) – An ex-boyfriend charged with murdering a Portage woman appeared in court on Tuesday, as prosecutors provided chilling details of the crime.

Dustin McCowan, 18, pleaded not guilty to the murder of his ex-girlfriend, Amanda Bach, 19, at a court hearing in Porter County, Ind.

Prosecutors said a neighbor might have overheard what was said early Friday morning, just moments after Bach was killed.

Witnesses told investigators they heard voices at McCowan’s home in Union Township last Friday. Bach’s body was found a day later. McCowan, 18, was charged Monday with murdering Bach by shooting her in the neck.

According to prosecutors, McCowan got together with Bach at his home on Thursday night. In a prosecution affidavit filed Tuesday, next-door neighbor Nancy Phillips said she heard voices outside between 1 a.m. and 2 a.m. on Friday.

Prosecutors said Phillips heard a female voice saying, “I can’t believe this is happening,” then heard a male voice saying “Amanda, get up!”

“Phillips advised that she heard what she thought was a male voice say ‘Amanda get up, come on get up, Amanda, get up,’” according to the affidavit. Phillips indicated that she heard that plea repeated about five times.

Amanda Bach, 19, went missing on Sept. 15, 2011. (Credit: Porter County Sheriff's Department)

Authorities found Bach’s body less than 300 feet from McCowan’s home, near a set of railroad tracks, but he was sticking to his story that he didn’t have anything to do with Bach’s death.

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McCowan’s defense attorney, Bob Harper, said, “Dustin says that he does not know what happened after she left the house and he’s told everyone that.”

But even those closest to McCowan said they believe he was responsible for Bach’s death. McCowan’s best friend, Brandon Hutchins, was also very close to Amanda, though he said they were not dating.

Hutchins and his family said they believe McCowan felt he was losing everyone he cared about.

Hutchins attended Tuesday’s hearing and said their friendship became strange after McCowan and Bach broke up after they were dating off and on for two years.

“He told her two weeks ago that he never wanted her to text him again, because she was coming between me and him,” Hutchins said. He said Bach wanted to date him, but he didn’t want to date her.

“I didn’t want to do that to Dustin,” Hutchins said.

His mother, Kim Howe, said she believes McCowan’s feelings of abandonment led him to murder.

“I think he did it over the friendship with him and Brandon. I think he just didn’t want anybody to come between him and Brandon,” she said.

“He loved Brandon like a brother. I mean … I didn’t have to give birth to him to be his mom. He was my son,” Howe added, choking back tears. “He’s destroyed so many people in the community.”

As police launched a massive search, Amanda Bach’s car was found a couple of miles away on Friday.

Bach’s father had told investigators that he thought Amanda and McCowan had been together Thursday night.

McCowan’s father, who is a Crown Point police officer, refused to allow a search of his property, authorities said.

Valparaiso investigators began a search nearby and found Bach’s body Saturday afternoon on railroad property about 300 yards from the McCowan home.

She had been shot in the neck, and the coroner said she would have died instantly. Bach had planned to attend Indiana University Northwest in Gary in January.

Authorities said McCowan was originally taken into custody by campus police at Indiana University Bloomington on Friday, where he was visiting friends. He was eventually transported early Sunday to the Porter County Sheriff’s office in Valparaiso.