DENVER — A Denver man was convicted Friday of murdering his girlfriend, nearly 10 years after her body was discovered inside a Denver-bound Amtrak train.

Angelo Valentino Mantych, 43, was convicted by a jury of his peers on one count of first-degree murder after deliberation for killing 28-year-old Marina Placensia while on-board an Amtrak overnight train that was headed to Denver from Wisconsin the morning of Sept. 1, 2016.

It took jurors about five hours of deliberation to reach the guilty verdict on Friday. Each was individually polled on their decision — and all of them confirmed their verdict.

Mantych, who was in court Friday for the reading of the verdict, hanged his head as it was read. He was scheduled to be sentenced on Jan. 9.

Meanwhile, Placensia’s family was overcome with emotion when they heard the guilty verdict. Outside of the courtroom, Placensia’s father told Denver7 the day felt like Christmas, New Years Eve, and his birthday rolled into one day.

Denver7 also spoke with a juror, who said the group was united in their decision and worked well together.

First-degree murder carries a mandatory sentence of life in prison without the possibility of parole, but the sentencing hearing will be an opportunity for those who loved Placensia to tell the judge how this loss has impacted their lives.

The two sides delivered their closing arguments on Friday morning.

Prosecutors said Placensia took the train that day with her four children, three of whom she shared with Mantych, to escape years of abuse and domestic violence. They argued that a “loss of control” served as motive for the killing. Prosecutors aimed to discredit the alternative explanations offered for Placensia’s death, pointing to what they said was clear evidence of a smothering.

Man found guilty of murder 9 years after girlfriend’s body was found in Denver-bound Amtrak train

The defense’s closing arguments centered around an apparent lack of commotion on the train on the night of Placensia’s death, saying it would be “almost impossible” for her to have been smothered for several minutes without their children noticing. Defense attorneys also called into question the integrity of an investigation that saw significant delays.

The case against Mantych is one Denver7 has been following since at least October 2023, when Mantych was charged with murder in connection with the crime.

An arrest affidavit obtained by Denver7 in the case showed the woman had several bruises on her body “consistent with an assault or struggle,” but investigators were not able to identify an obvious cause of death at the time. An autopsy later found blunt impacts to her head, trunk and extremities.

The investigation took several years to piece together, something the defense brought into question during closing arguments Friday.

“I think this case is such a great example of law enforcement just remaining committed to a case that they knew something happened, but we just had challenges with filing it, and so it’s a real testament to endurance and commitment by the entire team who’s worked on it since 2016,” said Assistant District Attorney Lara Mullin outside of the courtroom.

“It sends a very strong message to offenders who think that they can continue to perpetrate these types of crimes and violence on victims of domestic violence, that there will be justice sought and that we will not relent in our pursuit of that justice for victims of domestic violence,” said Amelia Sapp, the Chief Trial Deputy of Domestic Violence and Child Victims.

During the investigative process, one person told detectives that Mantych beat Placensia daily, and another said she had to go to a hospital multiple times for her injuries from the assaults, according to arresting documents. At least one of the reports obtained from the Racine Police Department and dated March 13, 2015, showed Mantych had punched Placensia’s ear several times, causing her to lose hearing in that ear. He was arrested and charged with assault after that incident.

On May 18, 2023, a doctor confirmed he believed Placensia’s cause of death was the result of asphyxia from suffocation and said her injuries were consistent with suffocation cases in both living and deceased patients, the affidavit stated. He said he believed her injuries were the result of an assault, including blunt force trauma and suffocation.

Mantych pleaded not guilty to the first-degree murder charge on June 21, 2024, according to court documents.

His trial began at the beginning of the month.

He did not testify in the trial and his defense team declined to comment on the verdict.

Editor’s note: If you or someone you know is experiencing domestic violence, help is available through Violence Free Colorado or the National Domestic Violence Hotline at 800-799-7233.

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