(CNN) A man suspected of killing missing Utah college student Mackenzie Lueck is expected to be charged with aggravated murder, authorities said Friday.

Lueck had not been seen since the early morning of June 17, when she took a Lyft to a park in North Salt Lake City, according to police.

Their investigation showed Lueck and Ajayi's phones were both in the park within a minute of each other, Brown said. This was approximately the same time that Lueck's phone stopped receiving data or location services.

Lueck captured on airport surveillance after landing in Salt Lake City June 17.

On Wednesday, as detectives searched Ajayi's home and property, Ajayi's neighbors told police they saw him using gasoline to burn something in his backyard on June 17 and June 18, according to Brown.

"A forensic excavation of the burn area was conducted, which resulted in the finding of several charred items that were consistent with personal items of Mackenzie Lueck," Brown told reporters.

Police also discovered charred material that was determined to be female human tissue, he said. The DNA of the human tissue was found to be consistent with Lueck's DNA profile.

A mattress that investigators have been trying to find has been located, police tweeted Friday night.

Suspect denied knowing victim

Ajayi, 31, was booked into the Salt Lake County jail Friday.

He denied knowing what Lueck looked like despite the fact that several pictures of her were found on his phone, Brown said.

CNN has been unable to determine whether Ajayi is represented by an attorney.

According to his LinkedIn account, Ajayi is a former Army information technology specialist and most recently worked for Goldman Sachs and Dell.

CNN has reached out to both Dell and the US Army for comment.

His neighbor, Tom Camomile, told CNN prior to Ajayi's arrest that he was a "smart guy" and a "computer geek."

Camomile said at that time it was "a little unnerving and unsettling" that Ajayi was connected to the case, "but you don't know anyone."

Ajayi's house is five miles from the park where the college student was last seen.

North Park police, whose jurisdiction includes the cities of Hyde Park and North Logan about 80 miles north of North Salt Lake, said Friday that Ajayi was the suspect in a 2014 sexual offense/rape case but the woman involved didn't want to pursue charges.

Student missing since June 17

Police previously said that Lueck, a 23-year-old University of Utah student, flew in to Salt Lake City International Airport on June 17 and texted her parents at 1 a.m. when she landed.

Airport surveillance footage showed her rolling her luggage through the airport in the early morning hours.

Lueck then took a Lyft to Hatch Park, the last place she was seen.

Salt Lake City Police Assistant Chief Tim Doubt told reporters Monday that the Lyft driver "stated that Mackenzie did not appear to be in any type of distress."

Police said Friday that all communications with Lueck's phone ceased around 3 a.m. that morning -- the same time they said Lueck left the park with Ajayi.

Phone records showed her last communication was with the suspect, Brown said.

Instagram account active after Lueck vanished

Lueck's Instagram account has been active since she disappeared. An account bearing the name "Kenzie Lueck" followed another Instagram user on or about Wednesday, CNN verified. That user has not replied to CNN's request for comment.

An image of Mackenzie Lueck taken from her Facebook page.

It's possible another individual accessed Lueck's account and is responsible for the activity.

The account belongs to Lueck, her sorority sister Kennedy Stoner confirmed. Lueck's friends have shared information regarding the account's activity with Salt Lake City police, Stoner said.

Investigators have been looking into the account's activity, along with the rest of the case's "digital footprint," Brown confirmed Thursday.

"This is a digital forensic investigation," he said. "This is covering computers, cell phones, IP addresses, URLs, texting apps."

'One of the most difficult phone calls I've ever made'

After Ajayi's arrest, Brown contacted Lueck's parents, who he said were "devastated and heartbroken by this news."

"This is one of the most difficult phone calls I've ever made," Brown said.

Lueck's uncle read a statement on behalf of the family at Friday's news conference in which they thanked law enforcement for their effort in the investigation.

"They're also grateful to her community, her friends and others around the nation who have supported the investigation," the statement said.

Friends and family told police that it was unusual to not hear from Lueck. One friend and sorority sister told CNN affiliate KSL that Lueck had also missed exams.

"She's extremely dedicated," Ashley Fine told the TV station. "She would never miss her midterms or anything like that.

"She hasn't been home. She didn't show up to work, or anything."

The University of Utah issued a statement saying school officials were saddened to learn of the death of Lueck, who was to graduate next spring with a degree in kinesiology.

"The death of Mackenzie Lueck is devastating news," university President Ruth V. Watkins said. "On behalf of the university, I express our heartfelt sympathy to the family, friends and classmates of Mackenzie during this very difficult time."

Correction: This story has been updated to indicate formal charges have not yet been filed.