One of the few clues in Audrii Cunningham’s disappearance might be a red “Hello Kitty” backpack she was supposed to carry to school with her last week.

The 11-year-old vanished after she left to catch the school bus Thursday morning in the small, east Texas town of Livingston, about 70 miles northeast of Houston.

Audrii never made it onto the bus, the Polk County Sheriff’s Office said. But “a small backpack that likely belonged to a child was located near the Lake Livingston Dam,” the sheriff’s office said.

Now, a man who was arrested for an unrelated aggravated assault case is believed to be “one of the persons of interest involved in the disappearance” of Audrii, according to the sheriff, who said the man lived behind the family’s home. But the girl’s whereabouts remain a mystery.

Here’s what we know about the case:

Sheriff says a criminal investigation is underway

Polk County Sheriff Byron Lyons said at a Monday news conference that investigators are working to determine what crime may have been committed.

“Once we get all that evidence put together (we will determine) what charges will be filed,” he said.

Lyons said he has not given up hope the girl is alive.

Investigators have identified a 42-year-old man “as one of the persons of interest involved in the disappearance” and believe his dark blue 2003 Chevrolet Suburban was involved, the sheriff’s office said.

The sheriff’s office arrested the man Friday on an unrelated charge of aggravated assault.

The sheriff told reporters the man is a friend of Audrii’s family.

The person of interest lives in a trailer on the family’s property and sometimes took Audrii to the bus stop, the sheriff added. The man has been talking to investigators but hasn’t acknowledged whether he dropped her off that morning, according to Lyons.

Authorities asked for people who have video of a road near the lake from Thursday morning between 6:30 a.m. and 8 a.m. to submit it as investigators build a timeline.

Lyons added other people that have been looked at during the investigation and said no one has been ruled out as a possible perpetrator.

Audrii never made it to school

Audrii was last seen near her home on Lakeside Drive, the Polk County Sheriff’s Office said.

The home is close to Lake Livingston – “one of the largest reservoirs in the state, with 83,000 surface acres,” according to Texas Parks & Wildlife. “The lake is an impoundment of the Trinity River, and provides water for the city of Houston and other East Texas cities.”

According to the sheriff’s office, “Audrii should have caught the school bus at her neighborhood bus stop; however, school officials reported to the Sheriff’s Office that the school bus did not pick Audrii up, nor did she report to school.”

Audrii’s family reported her missing after she didn’t return home from school. A multi-agency search ensued, including the Texas Rangers, the Texas Department of Public Safety, the Livingston Police Department, the Texas State Guard and several local fire departments, the sheriff’s office said.

Her backpack may have been found

On Friday, “a small backpack that likely belonged to a child was located near the Lake Livingston Dam,” the Polk County Sheriff’s Office said.

Previously, authorities said Audrii was carrying a bright red “Hello Kitty” backpack.

But the investigation “remains active and ongoing,” the sheriff’s office said.

‘This is the stuff you see on TV’

Audrii’s mother, Cassie Matthews, said her heart always broke for the parents of missing children whom she heard about on the news.

“This is the stuff you see on TV, and you can only imagine what the family and the parents are going through,” Matthews told CNN affiliate KPRC.

Now, she’s one of those parents.

“There’s not words for it. There is not one feeling you feel. It’s a roller coaster. You are broken, you are mad, you are empty. And right now, I am empty,” Matthews said.

“She has so many opportunities ahead of her, and she deserves every right to be able to reach those opportunities.”

Audrii is White with blond hair and blue eyes. She stands about 4 feet, 1 inch tall and weighs about 75 pounds, the sheriff’s office said. The girl was last seen wearing black pants, a black hoodie with white lettering and black high-top tennis shoes.

Anyone with information in this case is asked to immediately contact the Polk County Sheriff’s Office at 936-327-6810 or submit an anonymous tip online.