CORTEZ, Colo. -- The charge against a New Mexico man accused of killing a 16-year-old transgender Navajo teen was upgraded to first-degree murder because of evidence that a robbery was involved, the prosecutor said.

But Montezuma County District Attorney Joe Olt declined Monday to call the bludgeoning death of Fred C. Martinez Jr. a hate crime.

Shaun Murphy, 18 (pictured, left), a Farmington, N.M., oilfield worker, was originally charged with second-degree murder.

Martinez (pictured, right) has been described as being "two-spirited," the Navajo way of saying he had elements of both male and female genders. His family and friends say the transgender youth was comfortable with his identity.

Martinez' death has received a lot of attention from local and national gay-rights groups, which say it appears he was killed because he was transgender.

Olt, though, said he wasn't saying the crime was premeditated or hate-motivated. He said he upgraded the charge to first-degree murder because the crime allegedly occurred in conjunction with robbery, another felony offense.

The crime carries a possible death penalty or life prison sentence.

Martinez' mother, Pauline Mitchell, called the charge of first-degree murder more appropriate. She said in a statement she hopes "this takes us a step closer to justice for the death of my son."

Martinez' bludgeoned body was found June 21 near Cortez. Olt said the police investigation turned up evidence that Martinez was robbed before or after his skull was fractured with a blunt object.

Olt has never prosecuted a murder case. He is consulting with the state attorney general's office and other prosecutors about the case.

"We care very much about this case. We allege that Martinez was brutally murdered," Olt said.

Murphy's preliminary hearing was scheduled for Sept. 7.