KNOXVILLE, Tenn. — Tennessee gun control advocates say they will continue their push for stiffer penalties against adult gun owners who leave loaded firearms within reach of young children after a 13-year-old Knoxville boy was charged Wednesday with first-degree murder in the shooting death of his 12-year-old brother.

"It all comes down to accessibility," said Beth Joslin Roth, executive director of Safe Tennessee Project, a grassroots organization working to reduce gun violence. "Gun violence is a very complicated issue. ... But these incidents involving children with access to loaded guns are not complicated."

Police said the teen and his younger brother were the only two people in the family's North Knoxville home at 2101 Needham Drive when the shooting was reported at 7:14 p.m. Tuesday.

Knoxville Police Chief David Rausch declined to offer an explanation as to how the boy gained access to the weapon and said the motive remains under investigation. Authorities have not released the name of either boy.

Before this week's shooting, Safe Tennessee Project documented 39 incidents across the state since January 2015 involving children who injured or killed themselves or another person with a gun. The results include 14 deaths, 23 injuries to children and two injuries to adults.

The majority of the shootings were deemed unintentional.

Others were not, including the death of 8-year-old MaKayla Dyer, of White Pine, Tenn., who was fatally shot Oct. 3, 2015, by her 11-year-old neighbor. Authorities said the boy, Benny Tiller, grabbed his father's shotgun and shot MaKayla once in the chest from his bedroom window.

In February, a Jefferson County Juvenile Court judge found Tiller delinquent by reason of first-degree murder and ordered him held in state custody until age 19. No one else has been charged.

In March, a state House committee voted down a bill, known as MaKayla's Law, which would have held gun owners criminally responsible for leaving a firearm loaded and unlocked if it fell into the hands of a child under 13. A violation would have been a class A misdemeanor; a class E felony if the child injured someone; and a class C felony if the child fired a shot resulting in death.

Committee members killed the bill after a National Rifle Association lobbyist voiced opposition.

Joslin Roth said the bill's sponsors plan to resubmit a simplified version of the measure in the coming legislative session as an amendment to state law's existing reckless endangerment statute.

She noted that while the bill would establish a level of legal uniformity, Tennessee prosecutors still have discretion in holding adult gun owners accountable in such cases. In May, Kelly Pittman was indicted by a Hamilton County grand jury on charges of criminally negligent homicide and child neglect in the death of her 3-year-old son, Gavin Pittman, who accidentally shot himself in the head after finding a loaded pistol in the unlocked glove box of the family minivan.

Attorney Gregory P. Isaacs appeared at a hearing Thursday morning in Knox County Juvenile Court with the parents of the 13-year-old boy charged in his brother's death.

The parents left the courthouse after the hearing without commenting. Isaacs declined comment outside the proceeding.

"Children, especially children this age, are very impulsive," Roth said. "They don't always exercise good judgment. They're emotional. Access to firearms ... can lead to them taking an action they will regret for the rest of their life.

"What's most frustrating about it for me is ... these incidents are 100 percent preventable."