TAMPA, Florida (CNN) -- Growing more anxious with each passing day, the parents of a suspected kidnap victim in Florida are increasing the reward for their son's return from $10,000 to $250,000.
"We want him back. We love him. We miss him," Thomas Wiles said of his son, his eyes filling with tears.
Robert Wiles, 26, of Lakeland, Florida, disappeared April 1. He was last seen at National Flight Services at Lakeland Linder Regional Airport, where he worked for his family's flight maintenance company as a business development manager.
Two days later, his father received a ransom note demanding money. The FBI won't say how much, nor will it release the name on the note because it doesn't want to spark copycats. The bureau only describes the name as unique and specific.
"It threatened Robert with harm if the terms were not complied with exactly as they say," Thomas Wiles said. "I think when I first looked at it, I felt it wasn't for real, but then subsequently I realized it was for real, and I'd best comply with the terms." Watch the parents appeal for help finding their son »
The FBI will say only that the family tried to follow instructions but heard nothing more -- and that the case is unusual.
"These types of kidnappings [of adults for ransom] are extremely rare," said Steve Ibison, Tampa FBI special agent in charge. He said he had no statistics.
"We have some individuals of interest, we'll call them at this point, that we are aggressively pursuing," Ibison added.
The FBI is tracking leads in several states and outside the U.S., Ibison said.
By offering $50,000 for their son's safe return and another $200,000 for information leading to the arrest and conviction of those responsible, Wiles' parents said they're hoping to spark leads.
"We're willing to work with them and do anything to get him back. We just want Robert back," Thomas Wiles said.
The Wileses own plane-maintenance companies in Ohio, Florida, Texas, Missouri and Canada.
Robert Wiles is a pilot who sometimes would fly a company plane to meetings throughout the U.S. and Caribbean.
Co-workers at the Florida airport facility said they're baffled by the kidnapping. They said they can't think of a reason for kidnapping the young man.
"It's very, very upsetting to myself and other staffers here," Chad Mountcastle said. "We want Robert to get back to work and be safe."
Co-worker Gerry Grieff said, "Robert didn't make enemies. I don't know of any enemies he made in the business."
His mother, Pam Wiles, said there is "no way" her son was involved in anything illegal.
His friends and family said Wiles wouldn't simply run away. They also said Wiles, who is not married, would never abandon his family.
The FBI offered a message to those who might be holding Wiles.
"They should come forward, because the idea is to bring Robert back home safely," Ibison said. "It's gonna be a lot better on whoever is involved if he comes back safely and soon."
Wiles' father put it simply: "Please let him loose." E-mail to a friend
CNN's Susan Candiotti, Rich Phillips and Mike Miller contributed to this report.