(CNN) Volunteers and law enforcement combed the side of a highway on Thursday looking for evidence in the disappearance of a missing Wisconsin teenager whose parents were found dead in their home this week.

But the search along Highway 8 in Barron County, Wisconsin, didn't turn up anything of value, according to Barron County Sheriff Chris Fitzgerald.

Hours earlier, Fitzgerald asked for 100 volunteers to help in the routine search for evidence that could be related to the case as the search for Jayme Closs entered its fourth day.

The 13-year-old was likely at her family's Barron home, located on Highway 8, when her parents were shot dead, and she vanished moments later, investigators believe. Her whereabouts and safety are still in question.

"We believe Jayme was in the home at the time of the homicides and we believe she's still in danger," Fitzgerald said this week.

Thursday's search took place about 3 miles from the family's home, according to CNN affiliate WCCO

Since authorities received a cryptic 911 call and discovered the bodies of Jayme's parents in their home near the town of Barron early Monday, investigators have received more than 800 tips and have not confirmed any credible sightings of the girl.

But the sheriff said he has a "100% expectation that she's alive."

#FBIMilwaukee needs your help, as the search continues to bring home 13 year old Jayme Closs, now missing & endangered after her parents were found dead in their home in Barron, WI early this week. Call the tip line 1-855-744-3879.#FindJayme pic.twitter.com/rNELlQKJb6 — FBI Milwaukee (@FBIMilwaukee) October 17, 2018

An Amber Alert was issued Monday for Jayme and several law enforcement agencies have joined the search.

Motive in parents' death is unclear

Deputies are also trying to solve the killings of Jayme's parents, James Closs, 56, and Denise Closs, 46, in the small city of Barron.

During a 911 call shortly before 1 a.m. Monday, the dispatcher heard a disturbance in the background. But no one spoke directly to the dispatcher, Fitzgerald said.

When deputes arrived to the home less than four minutes later, Fitzgerald said, no one was in sight and no vehicles were in the immediate area.

Closs' parents were shot and their deaths have been ruled homicides, Fitzgerald said Wednesday. No gun was found at the scene, he said.

It's not clear how long James and Denise Closs had been dead when their bodies were discovered Monday.

On HLN's "Crime & Justice" Wednesday night, Fitzgerald told host Ashleigh Banfield that deputies had recovered the cell phone from which the 911 call was made.

Fitzgerald said additional agencies, including the FBI, are involved. "They are the experts in breaking down 911 tapes, looking at our phones, and taking care of all evidence in that manner," he said.

Authorities said they have determined whose cell phone the call came from, but declined to identify the owner.

Investigators also believe Jayme was at home during the shooting based on details from the 911 call and evidence from the home.

"Is it a random attack or a targeted attack? I don't know that answer," Fitzgerald told reporters. "That's why those leads are so important."

Joan Smrekar, who lives next door to the Closs home, told Banfield she heard two shots a couple of seconds apart just after 12:30 a.m. Monday.

"It was just, 'bang' and 'bang,'" Smrekar said.

Relatives wait in agony

Seara Closs said she wishes she were the one endangered, not her cousin Jayme. Seara wrote an open letter to Jayme on Facebook.

"I'm going thru our family pictures, worrying sick about you :( wishing we could trade places just to get you home and out of harms way," Seara Closs posted

In her post, Seara reminded her cousin that her family -- including her slain parents -- love her dearly.

"Grandpa Jim (James) Closs, your Momma Bear, Denise Closs and your very own night [in] shinning armor, your Daddyo Jim JR Closs ... love all of you!" Seara Closs wrote.

Authorities said they don't believe Jayme Closs ran away.

Barron Area School District administrator Diane Tremblay said Jayme, a member of her school's cross-country team, is a "sweet girl who is a loyal friend and loves to dance."

During a recent school assignment, Jayme was asked what she would do with $1 million, Tremblay said. Jayme wrote that she would "feed the hungry and give the rest to the poor."

Both James and Denise Closs were long-time employees of the Jennie-O Turkey Store in Barron, Jennie-O's parent company Hormel said.

"Our thoughts are with the Closs family and the entire Barron community," said a statement from Jennie-O Turkey Store officials.

"This is a difficult time for our entire team and we are mourning this loss and are still processing this terrible tragedy. We are also hopeful for the safe return of their daughter, Jayme, and are keeping her and the Closs family in our thoughts."

Jayme Closs is 5 feet tall, weighs 100 pounds and has green eyes and blond or strawberry blond hair, the sheriff's department said. Anyone with information can call the tip line at 1-855-744-3879.

Correction: A prior version of this story incorrectly spelled the missing girl's last name.