The FBI is leading the hunt for a Missouri dad — described by family members as "delusional" — who methodically planned and carried out a scheme to snatch his three young kids and smuggle them out of the country, FOXNews.com has learned.

Authorities believe Joel Hundley, 34, took his children — ages 10 months, 2 and 4 years old — from the family's house on Feb. 19, and flew with them to El Salvador.

Family members told FOXNews.com that they believe Hundley is engaged in questionable sexual activities, and that they fear the children could become victims of a human trafficking or child prostitution ring.

"Either he's completely out of it … thinking he's only protecting his children by kidnapping them, taking them across borders and hiding them from authorities, or the other, which I'm really concerned about," Hundley's sister, Shelley Hundley, told FOXNews.com on Tuesday. "Definitely my main concern is, he has been involved in some online, really sketchy, really troubling sexual trade and tourism."

She added: "It's to me, inevitable … that they will end up in prostitution, whether it's five years or three months [from now] or whatever, they will end up in prostitution. There's no question. It will end up being sexual exploitation of children."

Shelley Hundley said her brother's scheme culminated Feb. 19 when he told his wife, Heather, to go out and enjoy a day with friends or at a local spa. A few days before, Shelley Hundley said, Joel was friendlier than usual and even did the kids' laundry.

When Heather was leaving the house, the youngest child, 10-month-old Alexandria Hundley-Mondello, who was still breast-feeding, apparently was fussy and Joel suggested to Heather that she take the child, Shelley Hundley said.

Heather didn't.

When she returned home later that day, she found the kids and Joel had vanished. Heather Hundley said her husband left her a note saying her oldest daughter was vomiting blood, and that a neighbor had taken him and the child to the hospital. But she searched all area hospitals and found no trace of either of them. She asked a friend to come back and do a walk-through of the house with her.

"We realized all of their clothes were missing, all their toys were missing, and he was gone with all their suitcases. He had disappeared," Heather Hundley told FOX News' Greta VanSusteren Tuesday night.

Joel has sent at least one e-mail to the Hundley family since his departure, in which he mentioned that he took the kids to get away from the "violence" at home, Shelley said. But the children were in no danger at home, she insisted.

In that e-mail, Joel said he was sorry he had to take them, but didn't give any real motivation for why he did it, she said. Shelley said Joel had told friends about his plan to take the children.

"The children are not being abused by their mother, there's not any kind of reason … he has very delusional issues," Shelley said.

Shelley said Joel has a history of reading sexually explicit material and participating in sexual chatrooms on the Internet. He also engaged in what she called "group sexual activity slime," even taking at least one child to a group activity on an occasion.

"I know of one instance at least there was some really disturbing sexual things going on ... I don't know what happened to the child, I don't have any proof the child was participating, but he [Joel] definitely doesn't have a boundary line," Shelley said.

Authorities believe Joel may be in El Salvador, since that is his last known location. He purchased a ticket for a flight that landed in that country.

The Lee's Summit Police Department in Missouri has turned the case over to the FBI. Jeff Lanza, a FBI spokesman, told FOXNews.com on Tuesday that authorities are in working on getting a criminal warrant for Joel Hundley's arrest.

Depending on the country, the United States can either get a warrant for someone's arrest or work through the State Department for a more diplomatic way to extradite a suspect or question him, Lanza said.

"At this point, we're working on the resources we have to get the children back safely," said Lanza, adding that the FBI Office of International Relations is also involved in the investigation.

Joel and his family grew up in Colombia and he is fluent in Spanish, Shelley said. That, along with El Salvador's extradition laws, may be a reason he fled there, she said.

"I think that he probably figured that out — that it would definitely be better if he were somewhere else" other than the United States, she said.

The oldest child, Julianna Hundley-Mondello, who is 4, was diagnosed with kidney malformation and urinary-tract problems just before she was taken from her father. Doctors said there was a 50 percent chance she needed surgery before she left. The third child, Elijah, is 2 years old.

Shelley Hundley said she had been trying to convince Heather to divorce her brother for a long time, particularly after she learned Joel at one time held a knife to his wife's throat, she said. The children also have said he hit her and pushed her down the stairs, Shelley said. He was known to be verbally abusive to her as well.

Heather Hundley filed for divorce soon after her children disappeared. She had threatened to do so before, but had never followed through on it.

"It takes a lot for a sister to say, 'take the kids away from my brother' … I had a very strong fear that this would happen, exactly," Shelley Hundley said.

The Hundleys had lived in Lee's Summit for one year.

On Sept. 22, 2006, the Lee's Summit Police Department responded to a domestic disturbance call from Heather Hundley, who said her husband was using physical force — but no reports of a weapon — on her.

Joel Hundley had fled by the time police arrived, however, and Heather would not press charges. The case was forwarded to a domestic violence detective and sent to the prosecutor, but went no further because of Heather's reluctance to prosecute.

Shelley Hundley said the abusive childhood she and her siblings had growing up no doubt contributed to his state today. Even though she has pleaded with him in the past to seek help and medication for his delusions and mental state, he has been resistant.

But right now, she thinks history is repeating itself and that Joel is perpetuating things that have been done to him in the past.

"I definitely do think that's the course going on here. Really, my only hope right now ... is that he himself will really realize that and see he's made a mistake," she said, pleading with him. "Take the children to the embassy. The children are not compatible with the sexual perversion lifestyle he enjoys. If it's money, tell us how much money it is."

She added: "I would hope he would just come to his senses and realize the kids are in no more danger than they are right now … [the situation] is irrevocable for him, it doesn't have to be irrevocable for the kids."

Anyone with information on the whereabouts of the children are asked to call the FBI in Missouri at (816) 512-8200.