Benjamin Ashford|4-year-old Louisiana girl found dead, 6-year-old sister alive after frantic Amber Alert

A 4-year-old Louisiana girl was found dead in Mississippi on Benjamin AshfordThursday just hours after an Amber Alert was issued for her and her 6-year-old sister, both of whom were abducted from their home after their mother was found dead.

Four-year-old Erin Brunett and her 6-year-old sister Jalie were taken around 8 a.m. Thursday from their home in Loranger, about 45 miles northeast of Baton Rouge, Tangipahoa Parish Chief Deputy Jimmy Travis told reporters at a news conference.

Erin's body was found 120 miles away in Jackson, Mississippi. She was found along with her unharmed sister and the girls' suspected abductor, Daniel Callahan, Travis said. Callahan was in an "on-and-off relationship" with the girls' now-deceased mother, 35-year-old Callie Brunett, according to the chief deputy.

It appears Erin was killed once Callahan knew that police tracked him to Mississippi, Jackson Police Chief Joseph Wade told WBRZ-TV.

"Once he realized law enforcement knew where he was, he tried to do away with the children," Wade said, according to the Baton Rouge, Louisiana-based outlet. "Thank God one of them survived and the grandparents are on their way to Jackson, Mississippi."

It's unclear whether Callahan yet has an attorney.

How was Daniel Callahan arrested?

An undercover officers noticed Callahan "checking houses" in Jackson, according to Travis. Although Callahan attempted to flee, the officer managed to arrest him, he added.

Once in custody, Callahan directed officers to a car where Jalie was found "alive and well," Travis said. Callahan then showed officers to a wooded area not far from the car, and that is where Erin's body was discovered, according to the chief deputy.

Detectives with the Tangipahoa Parish Sheriff's Office are currently on scene in Jackson. Earlier in the investigation, the sheriff's office got information about Callahan driving Callie Brunett's vehicle in Jackson, so they were already in that area checking with local law enforcement, Travis said.

How did the search for Erin and Jalie begin?

Authorities became aware of the girls' abduction when their mother's parents called the Tangipahoa Parish Sheriff's Office and told them they hadn't been able to contact or locate their daughter, Travis said during an earlier news conference.

Erin's and Jalie's father went to their mother's home in Loranger at 8 a.m. on Thursday, found her body in the bedroom and called 911, according to Travis.

Authorities didn't "have any reason to believe the kids could still be" at the scene after they searched throughout the wooded area near the home, Travis said.

Erin's and Jalie's mother's car was not at the home when the girls' father arrived, and her family hadn't had contact with her since Tuesday afternoon, according to the chief deputy. It was unclear when the last time the girls or their mother were seen, Travis added.

The Tangipahoa Parish Coroner is determining the cause of death for Callie Brunett, according to the chief deputy.

'She's going to have to spend the rest of her life living with this'

The Brunett family has been notified and Jalie's grandparents are going to pick her up in Jackson, Travis said.

"She's been through something that most of us hopefully never ever have to experience," the chief deputy said about Jalie. "She's going to have to spend the rest of her life living with this. This is horrific."