Prosecutors say a former Boulder, Colorado, police officer shot and killed an elk while on duty so he could mount its head on a wall as a trophy.

During opening statements Wednesday in the trial of 37-year-old Sam Carter, prosecutors showed pictures of the head mounting process.

Carter's defense attorney argued he "euthanized" the animal because it had become too domesticated and was aggressive. He said he did it in the interest of public safety and never tried to mask his actions.

Another police officer who helped Carter, Brent Curnow, pleaded guilty to one felony and four misdemeanor charges and was sentenced in September to one year of probation and two months of house arrest.

Residents familiar with the elk had named him "Big Boy."