(CNN) Two teens in Utah and Alabama allegedly killed their mothers and siblings in separate incidents, authorities said, leaving their communities shocked in what appear to be unrelated cases.

The killings in Utah happened on January 17, and were the first in nearly two decades for Grantsville, which has a population of 11,000. Days later, on January 22, the Alabama teen was accused of targeting his family members.

Here's what we know:

Alabama teen allegedly killed mom and brothers -- then went to school

An Alabama teen allegedly killed his mother and twin 13-year-old brothers before going to school, authorities said.

Landon Durham, 16, faces a capital murder charge after the three were found dead of stab wounds Tuesday inside a house in Munford, the Talladega County Sheriff's Office said. He is being charged as an adult.

The victims were identified as Holli Christina Durham, 36; and Branson and Baron Durham, both 13.

The suspect allegedly killed them sometime Tuesday morning, then went to school, Talladega County District Attorney Steve Giddens said. He was arrested Wednesday morning at a store without incident, CNN affiliate WBRC reported.

Authorities have not released a motive for the killings. It's unclear whether Durham has an attorney.

Landon Durham, right, is shown with his mother and twin brothers in this photo.

Utah suspect skipped school to kill mom and siblings, police say

In Utah, a 16-year-old skipped school and killed his mother and three siblings on January 17 as each returned home, officials said this week.

Colin Jeffrey Haynie is being charged as an adult in the killings along with the attempted murder of his father, prosecutors said Wednesday. He faces four counts of aggravated murder and one count of attempted aggravated murder.

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Haynie shot his mother and sister multiple times as they arrived home around 1 p.m., Tooele County Attorney Scott Broadhead said. Consuelo Alejandra Haynie, 52, had picked up Maylan, 12, from school and the suspect opened fire when they got into the house, authorities said.

He then shot his sister Alexis, 15, multiple times after she came home from school, then later in the afternoon shot his brother Mathew, 14, according to the charging document.

"The victims all had gunshot wounds in their heads, necks, and/or shoulder areas," the document says.

At around 6:15 p.m., when his father arrived home, he shot him in the leg before the victim overpowered the teen, authorities said. "This happened over a five-hour period, piece by piece by piece by piece," Broadhead said.

A neighbor came to the home, called 911 and took the father and the suspect to a hospital. The teen confessed to the killings during the call and was arrested at the hospital, Broadhead said.

The teen does not have an attorney. His first court appearance is Monday.