The handgun was in the home "illegally," Young said without elaborating.

The shooting happened about 10:30 p.m. (11:30 p.m. ET) Monday,according to CNN affiliate WAFF. The accused teen called 911 around that time to report that he was in the basement when he heard gunfire upstairs, WAFF and fellow affiliate WHNT reported.

He repeated the claim to deputies who arrived in the family's driveway around 11 p.m., Young said.

"He said that he ran out the door, and there was very little other information given," Young said, adding that the teen was later taken in for questioning and "upon being confronted with some of the inconsistencies (in his story), he did admit to shooting the five family members."

The sheriff's office first reported early Tuesday that three of the five victims were confirmed dead at the scene and two were airlifted in critical condition. The office later confirmed the airlifted victims had died as well. Autopsies will be conducted Wednesday at the earliest, the coroner said

The boy is being held in a juvenile facility, Young said.

A Limestone County School District spokeswoman confirmed the teen attended Elkmont High School but referred additional questions to law enforcement.

"Limestone County Schools is deeply saddened by the tragic event," Karen Tucker said in an email. "We are concentrating our efforts in assisting our students, faculty and community in dealing with this tragedy. Additional school counselors and Limestone County Health Department Counselors are on site. These individuals will be available for as long as there is a need."

In a tribute on its Facebook page, the Paul Mitchell School in Huntsville, which offers classes in cosmetology, said John Sisk was an alumnus.

The shootings unfolded in a modest, one-story home on a large lot set back from a residential street. Elkmont, a town of fewer than 500 residents just south of the Tennessee border, is about a 45-minute drive northwest of Huntsville.

The town of Elkmont on Tuesday evening mourned the deaths. "We pray as a community for healing in this unprecedented situation and know that together we will persevere and eventually overcome," said Mayor Tracy Compton.