The suspect, who is 14 and is not being identified, is being held in a juvenile detention facility on five counts of murder, and he could be charged as an adult, according to the Limestone County Sheriff's Office.
The victims were identified by officials Tuesday as the teen's father, John Sisk, 38; his stepmother, Mary Sisk, 35; his 6-month-old brother, his 5-year-old sister and his 6-year-old brother. The names of the siblings have not been released.
"We are facing a tragedy that we aren't used to in Limestone County," said a statement from the sheriff's office. "This will have a ripple effect among the family, the community and county. It affects us all."
The suspect called 911 at about 11 p.m. Monday to report the shooting at his home in Elkmont, on the northern Alabama border. The teen met deputies in his driveway and told them he was downstairs when he heard shots fired upstairs, Limestone County Sheriff's Office Lt. Stephen Young said during a news conference Tuesday.
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But his story was full of inconsistencies, so he was brought to the sheriff's office to be interviewed, at which point he admitted to shooting his entire family, Young said.
He later led investigators to where he said he tossed the handgun on the side of the road near the house. Young said the gun was in the home illegally.
The teen's cousin, Daisy McCarty, who is John Sisk's niece, told NBC affiliate WAFF that the 14-year-old was a well-behaved boy until he started acting out about a year ago. She said he had broken into and vandalized his school and was in the habit of burning animals alive.
She said Mary and John told him last week that Mary was not his biological mother.
“I think he’s been having problems. ... That’s what bothers me because they’d seen it but they didn’t do anything about it really," McCarty said. "I think they didn’t talk to him enough to know what was really going on with him."
Mary Sisk, who was a special education teacher at Mountain Gap Middle School in Huntsville, wrote in her faculty profile that she was "happily married with a great husband and four beautiful children." She said she had been working in education for a decade.
Keith Ward, a spokesman for Huntsville City Schools said the district was "heartbroken." Additional counselors would be on hand to help students and staff process the loss, he said.
The Limestone County School District would also have counselors at their schools for "as long as there is a need," Limestone County Board of Education spokeswoman Karen Tucker said Tuesday.
Elkmont Mayor Tracy Compton said in a statement that the community was mourning "the loss of five of our residents, three of which were the most innocent among us, under the most brutal circumstances."
"There are many questions to be answered in the coming days but today, we unite as Elkmontians in prayer for the family, the community as a whole and also for the accused," Compton said.
Elisha Fieldstadt is a breaking news reporter for NBC News.