UPDATE December 18, 2019:

Missing Florida pilot Robert Wiles, who was believed kidnapped and held for ransom 21 months ago, now is believed to be dead, and a former co-worker is in custody, accused of his death, the FBI said.

Authorities arrested Stobert "Tobie" Holt Jr., 42, Friday in Orlando, Florida, after he arrived from Colombia, where he was on a business trip, according to Tampa-based FBI spokesman David Couvertier.

Holt faces state charges of first-degree murder, kidnapping, extortion, and intent to inflict death or serious bodily harm, the spokesman said. The arrest was made by police from Lakeland, Florida, the Polk County Sheriff's Office and the Florida Department of Law Enforcement, who were accompanied by FBI agents, he said.

A four-count grand jury indictment was unsealed Friday in Polk County, where Wiles, then 26, ran a branch of his family's international aircraft maintenance business, National Flight Service, at Lakeland Linder Regional Airport.

MORE: Arrest made in death of kidnapped Florida pilot, FBI says - CNN

June 8, 2017:

The son of a wealthy businessman remains missing. Investigators believe he's dead, and they also believe they know who killed him.

Robert Wiles, 26, is the heir-apparent at his father's business, National Flight Service, an Ohio-based company servicing privately owned aircraft.

"Robert was a very level-headed young man who seemed to be very much in tune to the future," said former deputy Florida state's attorney Cass Castillo.

But Robert prefers working at the company's Lakeland, Florida branch, where the sun always shines and the fishing is good.

"The father, Tom Wiles, wanted him to know the business from the ground up, so he was there in the sales position and trying to understand and to prepare himself for it," said Castillo.

But prosecutor Cass Castillo, who's handled this case from the beginning, says there is some friction in Florida.

"He did not have a lot of business experience," said Castillo. "So you had these different camps within the employees, and that created a level of friction between them and Robert."

And Robert's father back in Ohio, CEO Tom Wiles, has other problems to deal with in Lakeland: Namely, his top mechanic, Steve Lindsey.

"I think that Tom Wiles said that he was one of the best mechanics he's ever had work for him," said Castillo.

But Castillo says Tom Wiles fired Steve Lindsey for being intoxicated at work. Now Lindsey wants his job back.

"Steve Lindsey became very aggravating and agitated with Mr. Wiles and at some point even threatened him and made some rather overt threats toward Tom Wiles," said Castillo.

About a month later, a bombshell email arrives that will change the lives of the Wiles family forever. It's a ransom note from someone calling themselves "Group X."

"'We have Robert,'" said Castillo, quoting the note. "'If you hope to see him alive again, you must follow our instructions without deviation. Obtain an item of luggage of the appropriate size and place in it $750,000 in small unmarked untraceable bills. You should instruct someone that you trust to place the box unopened in your son's office. Remember we are watching everything. And if you think you can outsmart us, it will cost your son his life. Signed, Group X.'"

Robert Wiles had been scheduled to fly to Dallas for a trade show only days earlier. Now his family discovers that he never got on the plane, and has in fact disappeared.

"Tom Wiles immediately calls the FBI in Toledo," said Castillo.

And Tom Wiles says he will pay the ransom. FBI agents plan to leave the package in Robert's office per the demand.

"They were also very suspicious that it was bogus because of the instructions they were given and the location of where the money was going to be placed," said Castillo. "It wasn't the type of environment that gave very many options of escape."

But instead of $750,000 cash, they fill a box with coffee equal to the weight of the bills and place it on the desk in view of hidden cameras in the room. And at a nearby location they have a SWAT team ready to move at a moment's notice.

Tom Wiles also enlists his most trusted employee, Lakeland operations manager Toby Holt, to help out with the plan.

"Toby was absolutely aware of everything that was going on, as a matter of fact was helping the FBI by changing the tapes in there when he would come in in the morning."

But the box in Robert's office is never picked up. Still, the FBI keeps up the search for Robert, and they talk to his co-workers, including former employee Steve Lindsey, but with no luck, and no suspect.

The FBI determines that ransom note was sent from Robert's cellphone, which makes sense because anybody who has him would not use their own device, in order to avoid being traced.

But there's one thing the kidnappers didn't count on: The movement of Robert's phone can be traced through cellphone towers whenever it is switched on.

"As we tracked Robert's phone, we saw the path that it took," said Castillo. "Robert's phone got on the Polk Parkway, which is a toll road, which has a booth. We knew the time that the call was made and we know the time that it went through the booth."

And then, pay dirt. The agents identify a car carrying Robert's phone going through that booth two days after he goes missing. It belongs to someone who works at National Flight Service in Lakeland.

But it is not the disgruntled mechanic Steve Lindsey. The Wiles are shocked. It's someone they never would have suspected -- one of their most trusted employees: Toby Holt.

"You see Toby Holt holding a phone to his ear as he's driving through the toll booth," said Castillo.

Stunned agents now check the records for Toby's personal phone.

"When they start looking at Toby Holt's phone they realize that the phones are traveling together everywhere," said Castillo. "Many of the calls that were being generated were Toby Holt accessing Robert Wiles' voicemail."

Now agents dig into the past work relationship between Robert and Toby. And after talking to other employees, what they find is pure poison.

"Robert was very exasperated with his dealings with Toby and was just saying 'I can't take this much longer, he won't give me a chance, he won't listen to any of my ideas,'" said Castillo.

Investigators also learn that Toby was the last one in the building alone with Robert on the night he disappeared.

"We knew Robert was still there because of the emails that were being made, so we knew Robert was still at the facility, and Toby was there with him alone," said Castillo.

And Toby's phone also shows that he made a call that night to a nearby Home Depot only minutes before it closed. Shortly after that someone paid cash at the store for a hundred yards of plastic sheeting and two rolls of duct tape.

Investigators could not recover surveillance footage at the store because it had already been taped over. But they did track Toby's phone leaving the Home Depot right after it closed.

The FBI says there is a window of about three hours where they cannot account for Toby's whereabouts that night. He later says he went to a bar and restaurant and paid cash. But there are no receipts or witnesses at those places. And Castillo says that Holt, a married man, rarely pays with cash.

"During one of the nights he was going to go see a girlfriend he charged seven dollars of condoms at a drug store," said Castillo. "He charged everything. So for him not to be charging his dinner, that was out of character."

FBI agents arrest Toby Holt and charge him with kidnapping, extortion and murder.

It's one heck of a tough job for prosecutor Cass Castillo.

"There's really no evidence in the case about what happened to Robert Wiles," said Castillo. "It was indeed almost the perfect crime."

"The level of crime from which I could prove whether it was first-degree murder or second-degree murder or manslaughter became impossible because the evidence was unavailable," said prosecutor Cass Castillo.

But believe it or not, Castillo says the best witness is Robert Wiles himself.

"Robert Wiles would not voluntarily walk away from the family fortune and a business that he was so interested in developing," said Castillo. "Something happened to him."

The defense cites lack of evidence. But Castillo says the most damning evidence is Robert's cellphone, because everywhere it goes, there is Toby Holt.

"It is in many ways more conclusive and more persuasive than if I had three or four or five people say they saw him do it. Because that's not capable of being manipulated," said Castillo.

In the end the jury agrees. Holt is found guilty of manslaughter, not guilty on kidnapping, and sentenced to 30 years in prison.

"As far as the murder count is concerned then, there simply wasn't enough evidence because we don't know what happened," said Castillo.

But we wanted to get some answers from the man himself, so we went to see Toby Holt in prison, and we put it to him straight.

Did Toby Holt kidnap Robert Wiles?

"Absolutely not," Toby said.

Did he murder Robert Wiles?

"Absolutely not," said Toby.

But did he have a problem with Robert Wiles?

"There was some tension, but it was more to do with communication issues between Robert and not just myself, but with the other staff members as well," said Toby.

He admits he was at the Home Depot on the night of Robert's disappearance, but he denies buying that plastic wrap and duct tape.

"I called them up and asked them if they sold mini-blinds because we just three weeks earlier got a Home Depot credit card. So I was just going to send the facility guy down to get those," said Toby.

What about Robert's cellphone? How does Toby explain it showing up everywhere he goes in the days after Robert goes missing?

"I'm an avid follower of, you know, 'CSI,' and I know you can track cellphones, so it's not something I'm going to be carrying around with me," said Toby Holt.

Incredibly, he says someone with Robert's phone may have been following him.

"They may have been just following me for another reason. I suspect Steve Lindsey, but I don't know," Toby said.

Toby Holt's attorney also pointed the finger at Lindsey in court, claiming he desperately needed money.

"They never knew where Steve Lindsey was for the three days," said Toby.

Now we would love to hear Steve Lindsey's side of the story, but that's never going to happen, because Steve Lindsey died of natural causes before Toby Holt even went to trial.

And that leaves the whereabouts of Robert Wiles unknown. If Toby Holt is the killer, he surely knows where the body can be found. And according to former prosecutor Cass Castillo, he could possibly lessen his sentence if he cooperates.

"I don't have that information," said Toby. "And it's sad really because they [the Wiles family] have been convinced that I did it. And unfortunately somebody else did it, and they have gotten away with it."