Tristan Dilley, 14, was found dead in her bed on Sunday by her younger brother

It has been four days since 14-year-old Tristan Dilley was found dead in her Buna, Texas, bedroom. Soon after, the mystery surrounding her death would unravel, leading to the discovery of her secret relationship with a 19-year-old man.

Tristan was a freshman and cheerleader at Silsbee High School, described as a “positive” person with a contagious personality.

A funeral service for the teen will be held on Saturday at Adams EE Stringer Funeral Home in Buna, KJAS reports.

Here’s everything you need to know about the headline-making murder case:

1. Tristan Was Found Shot to Death in Her Bedroom Just Two Days Before Her Birthday

Tristan’s 13-year-old brother found her dead of gunshot wounds in her second-floor bedroom. She had been shot twice in the head with a .22-caliber magnum revolver two days before her 15th birthday. Her brother found her body fully clothed on top of the bed.

“She never saw it coming,” Jasper County, Texas, Sheriff’s Lt. Ryan Cunningham told PEOPLE. “You could tell from the scene there was not a struggle or a fight. She was comfortable with knowing the person.”

She wasn’t facing her killer when she died. The day before, Tristan attended a school dance and sleepover at a friend’s house.

Lt. Cunningham says detectives quickly ruled out a break-in in connection with Tristan’s death.

2. Police Discovered That Tristan Was in a Relationship with a 19-Year-Old Man

Shortly after Tristan’s death, police learned that the 14-year-old had been dating someone named “Adam,” who family and friends believed was 16. Her dad told authorities that Adam was “tall and lanky” and “supposed to be a high school basketball player,” Cunningham says.

However, police soon learned his true identity: Paul Audrey Adams, a 19-year-old nursing student at Lamar State College.

“Both kids were deceiving the parents,” Cunningham told PEOPLE. “He [Adams] knew he was too old to be seeing the child.”

Credit: GoFundMe

Investigators began searching Tristan’s phone and Facebook activity and learned that she and Adams had been planning meet-ups — coordinating around when Tristan’s mother would and wouldn’t be home.

“I called dispatch and had them start pinging his cellphone,” Cunningham said. “We put out a BOLO out for him in the surrounding area. We were able to get a general location of the cellphone and we learned of his address,” in nearby Vidor, Texas.

Officers searched Adams’ room and found a spent .22-caliber magazine casing lying in the middle of his bedroom floor.

Tristan Dilley
| Credit: Christopher Dilley/Facebook

“We don’t know if it was from something previous, but we knew it was probably the type of round used [to kill] Tristan,” Cunningham said. “We don’t know if maybe that it was a trophy-type thing. Sometimes they keep memories.”

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3. Investigators Believe Adams Killed Her

Authorities went to Adams’ home at 1:30 a.m. on Monday, but the man wasn’t home. They returned later at 10:30 a.m. and Adams called his mother while police were questioning her.

“The Ranger listened for a few minutes and took the phone,” Cunningham told PEOPLE. “He tried to get Paul to come in and speak to us, but he didn’t want to come in.”

Police said Adams admitted to the Texas Ranger that he had been at Tristan’s house the day she died — but said an intruder was also in the home.

“He gave a description of an older white male with a beard,” Cunningham recalled. “He said he hid in the shower and he could hear Tristan screaming, ‘Get off of me.’

Credit: Paul Adams/Facebook

“He said he heard two gunshots and heard the man running out of the house. He waited a couple of minutes and then found Tristan dead and he didn’t know what else to do, so he ran.”

Cunningham said investigators believe Adams was trying to deflect blame with the story.

“Why would you not immediately notify law enforcement?” he said. “It was pretty clear-cut. You could read between the lines.”

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4. Adams Killed Himself — and Left a Suicide Note

Authorities found Adams near a tent with his white ’94 Toyota by a canal just east of Vidor, Cunningham said. Police were about 150 yards from the vehicle and continuing to approach when they spotted Adams and saw him reach for a gun.

“As soon as me and my partner said, ‘I think he has a gun,’ we observed the suspect shoot himself in the head and collapse,” Cunningham said.

They found a suicide note at the scene, in which Adams maintained his innocence.

“[The note] was basically verbatim what he told the Ranger,” Cunningham said. “In the letter he said there was nothing he could do to change what happened to her and going to prison wouldn’t be worth it. He said, ‘I know people are going to think I did this.’ ”

Cunningham said that detectives learned that Adams bought several knives, dumbbells, duct tape, an axe, a machete, a hatchet and burlap sack just days before Tristan’s murder.

“It was a premeditated event,” he added.

Investigators also learned that Adams had downloaded apps to his cell phone to listen to police dispatch traffic in the wake of Tristan’s death, Cunningham told PEOPLE.

“I think he knew we were coming,” he said, “but I don’t think he realized it was going to be quite so fast.”

5. Tristan’s Family and Friends are Mourning the Loss of the Texas Teen

In her obituary, Tristan was remembered as an athletic student and “a very kind, sweet and compassionate young lady who was loved by so many.” An online fundraiser was launched to help pay for her funeral.

“She was very positive,” Silsbee High School Principal Paul Trevino told PEOPLE. “She had that type of personality that people wanted to be around her and hang out with her.”

Many were expected to gather at Buna High School on Tuesday to honor Tristan in a vigil.

“I love her and I thank her for being there for me and being a great friend and I will miss her so much,” KBMT reports.