After a series of legal setbacks, prosecutors decided to drop a first-degree murder charge against a New Mexico boy who police say shot his father in the back of the head with a shotgun in 2009.

District Attorney Lemuel Martinez said his office will be filing paperwork to dismiss the charge without prejudice, meaning it could be refiled if new witnesses or evidence come to light.

"This is one of the most difficult decisions that I've had to make while being the district attorney," Martinez told reporters Wednesday.

He said his office had little choice after a court ruled that testimony by the boy's then-6-year-old sister and his statements when he called 911 were inadmissible due to a state statute that applies to anyone under 13.

Prosecutors were unsuccessful in their effort to get the New Mexico Court of Appeals to consider allowing key evidence.

Authorities have said the then-10-year-old boy put a gun to the head of 42-year-old Byron Hilburn and killed him at their Belen home. The Associated Press is not naming the boy due to his age.

At the time, the boy was one of a handful of young children in the nation's history to face a first-degree murder charge.

Defense attorneys argued that the father was abusive to the boy and two siblings.

In a 911 recording from the night of the shooting, the boy tells a dispatcher: "I was just so over my head. I shot him in the back of the head. I got so angry at him."

The boy's defense team had planned to call dozens of witnesses to testify on his behalf about abuse he faced. The lawyers also planned to show the boy didn't fully grasp his actions that day.

In 2009, prosecutors and police were dealing with uncharted territory: The boy was the youngest person ever charged with murder in New Mexico.

Ballistic tests were done to prove the shotgun was used in the killing, and the father's fingerprints and DNA were compared to rule him out as the shooter. However, the boy's fingerprints and DNA were not compared to evidence found on the weapon.

"It's a little heart-breaking," Joe Portio, lead detective of the Belen Police Department, told Albuquerque television station KRQE.

Portio said at the time that investigators believed all the evidence they needed had been collected. There were challenges and misunderstandings because of the boy's age, he said.