The harrowing 911 calls from parents of Isabel Celis, whose remains were found last week.

At around 11pm on April 20, 2012, six-year-old Isabel Celis put herself to bed in her Tuscon, Arizona home.

At 8am the next morning, when her father went to wake up her, she was gone.

Isabel’s mother had left for work an hour earlier, and her father was the first to realise she was gone.

Last week, five years after she went missing, her remains were found in a remote area near her home. Despite the discovery, police say the mystery is not yet solved, and the investigation to find out what happened to Isabel continues.

Now, the chilling phone calls the young girl's parents, Rebecca and Sergio, made to 911 have resurfaced.

"I need to report a missing child, I believe she was abducted from my house," Sergio can be heard telling police.

"We woke up this morning and went to go get her up for her baseball game and she's gone.

"We looked everywhere around the house and my oldest son noticed that her window was wide open and the screen was laying in the backyard."

In a separate call to police, mum Rebecca said she "did not come and check on [Isabel]" before leaving for work.

"I should have come and checked on her," she said.

Isabel's disappearance triggered one of the largest missing child searches in the state's history.

Immediately after she vanished and authorities were alerted, more than 100 law enforcement officials, including FBI officers and U.S. Marshals, searched for her.

Police have refused to confirm what led them to Isabel's remains in March - with DNA testing confirming it was Isabel last week - but said investigations were still underway.

No arrests have been made in relation to her death, and police have not released the names of any possible suspects.

LISTEN: What it's like when a loved one goes missing. Loren O'Keeffe shares her story.