Rusty Arnold and his big sister Mary 'Rachel' Trlica were best friends. 

Despite their seven-year age gap, they shared an extraordinary bond as kids. Arnold idolized his older sister, describing Rachel as mature beyond her years. 

She married young and had already begun building an adult life, but still made time for her adoring little brother. But their bond came to an abrupt end when Arnold was just 11 and his elder sibling 17.

On December 23, 1974, during a routine trip to the mall to buy some last-minute Christmas gifts, Rachel, her friend Lisa 'Renee' Wilson, 14, and Wilson's boyfriend's younger sister, Julie Ann Moseley, 9, mysteriously vanished. 

Rachel's abandoned car and a letter addressed to her husband were the only traces left of the infamous Forth Worth Missing Trio.  

Fifty-two years later, the case still remains a mystery and one of the most baffling unsolved cases in modern American history. 

'You tell me a nine-year-old runs away two days before Christmas. Come on, don't even try it,' Arnold, now 62, exclusively told the Daily Mail. 

Arnold, who was only a young boy when his sister and her friends went missing, remembered how the carefree days of riding his bicycle with friends became time searching for his beloved sister.

'Me, my mom and my dad spent a lot of time looking,' he said. 'We went to different places, passing out circulars and posters with a reward.'  

Arnold recalled the letter addressed to Rachel's husband, Thomas 'Tommy' Albert Trlica, the day after his sister's disappearance.

'We just had to get away. We're going to Houston. See you in about a week. The car is in Sear's upper lot. Love, Rachel,' the letter read in part. 

Rachel's car, a 1972 Oldsmobile, was found in the parking lot of the Seminary South Shopping Center, located along South Freeway in Forth Worth, now called La Gran Plaza. But there was no trace of the girls. 

Rachel was in high school when she married Trlica, 21, who already had a young son from a previous marriage. In Texas in the '70s, it was not uncommon to marry at their age, according to Arnold.

The couple had only been married for six months before she disappeared. 

Police treated the case as if his sister and the other two girls were just teen runaways, Arnold claimed, despite the family's insistence that their theory did not make sense. 

'Don't tell me a 14-year-old girl runs away from home. Forever. Two days before Christmas. [She] had a great family life, everything was cool with her. I mean, no. That kind of crap don't happen now,' Arnold said. 

The girls had left to go buy Christmas presents shortly before noon that morning. Several witnesses reported seeing the trio at the mall during the day. 

By 6pm, the girls' concerned parents went searching for them at the mall. They stayed there all night into Christmas Eve.

'I think it should have been handled by the authorities more, and I think the authorities dropped the ball and botched the investigation,' Arnold continued.

When he was old enough, Arnold started searching for answers after losing faith in the police, enlisting the help of a private investigator who worked the case for 25 years. 

'The dives that we've done, the leads that we've chased, the rabbit holes, the red herrings. It's just been total chaos,' he said.

Between 2013 and 2018, Arnold found three cars submerged under a 30ft lake by their house.

He claimed that when police weren't interested in investigating his discovery, he  crowdfunded for a dive team to pull the cars out. 

Two of the three were removed, but the third vehicle, he said, was too difficult as it had disintegrated and injured one of their divers.

No sign of the Fort Worth Trio was found. 

Arnold claimed that in 2017, he had a forensic handwriting expert examine the letter from his missing sister. 

According to Arnold, the expert determined that the handwriting was likely not Rachel's. 

Arnold further claimed that when he brought the findings to the District Attorney's office, they were dismissed as inadmissible in court.

The Daily Mail has reached out to the Forth Worth District Attorney's office and Trlica for comment on the expert's findings.

Arnold also revealed that before Trlica married Rachel, he was engaged to Arnold's oldest sister, Debra.

Trlica and Debra, 19, were engaged for two weeks, but Arnold said his sister 'didn't take him seriously.' After they broke up, he started dating Rachel. 

Trlica was polygraphed and interviewed by police after Rachel's disappearance, but was eventually cleared as a suspect.

According to Arnold, Trlica is alive, remarried and living with ALS.

Arnold has heard all the different theories out there – some more bizarre then others. 

'I heard stories 30 years later of people telling me how it changed their lives. How it changed the way they thought about safety.

'There is an insane amount of stupidity out there. An insane amount of podcasts. Nobody can pronounce my sister's [last] name. It is just ridiculous.'

One circulating theory involved a security guard that may have talked to the girls outside before they vanished.

'I have weirdos and people not in their right mind come tell me things that happened to them in the mall. I had one guy tell me that he was molested at the mall by one of the security guards,' Arnold said. 

He also revealed that 'a lot of people' thought his sister Debra, now 70, was involved. Arnold admitted that Rachel's vanishing drove a wedge between them that lasted 20 years.

'It's not that crazy of an idea, but that's something that you'd have to take up with her. And guess what? She ain't talking to nobody.'

Arnold and his mother, Fran Langston, now 87, are very close and often talk about Rachel. 

'At family get togethers we always talk about how Rachel should be here and her kids  and her grandkids should be here,' he said.

One of his favorite memories, was going to McDonalds with Rachel, driving to different locations to collect special promotional prizes.

If a customer could recite their Big Mac jingle in four seconds, they would get the prize.

'All beef patty, special sauce, lettuce, cheese, pickles, onions, on a sesame seed bun – that is the Big Mac,' he laughed. 

His sister also taught him how to play his first few guitar chords, a skill he has used in adulthood to write his own music. 

One of the songs he wrote is called In Memory of You.

'It took me 20 years to write this song,' Arnold said. 'I know this may sound stupid but I didn't go into a man cave to write the song. I had to live to write it. 

'I can't just go and make up lyrics – they had to come out naturally.'

Fort Worth Police Department spokesperson Buddy Calzada said the case is still an 'active investigation.'

'We are still looking for somebody that may have evidence that they are holding back,' Calzada told the Daily Mail. 

'The individual that did it or maybe someone that they've talked to. So if there are any key pieces or components that might be available, we ask the public to reach out to us on that.' 

Arnold to this day is committed to finding who was involved with his sister's disappearance and has been working on trying to prove it.

An anonymous donor has put up a $50,000 reward.

'I am just hoping that the reward that we've offered will open some eyes, and some people will start talking. 

'Somebody out there knows something. They're here. They're alive today, you know, and maybe the $50,000 will jog their memory.'

He asked anybody with information leading to the arrest, conviction or location of the bodies to message him on his private Facebook page.