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A Florida woman was arrested on suspicion that she killed her young son who went missing in Nevada more than 30 years ago, authorities said Monday,

Amy Elizabeth Fleming, 60, was picked up by U.S. Marshals and officers in Boca Raton, Florida, on Jan. 29 before she was turned over to the North Las Vegas police over the weekend.

Fleming and her then-fiancé Lee Luster were awaiting trial for alleged child abuse against her son, Francillion "Yo-Yo" Pierre, when she told Nevada police in August 1986 that she lost the boy, then 3 years old, at a swap meet, North Las Vegas Police Chief Pamela Ojeda said at a Monday press conference.

"The majority of witnesses at the swap meet that day reported never seeing Pierre with Fleming or Luster," Ojeda said. "The neighbors of Fleming and Luster also told investigators that Pierre had not been seen for weeks."

Amy Fleming was charged with murder in the death of her 3-year-old son, Francillon Pierre, in 1986. National Center for Missing and Exploited Children / Palm Beach County Sheriff's Office

The couple pleaded guilty in 1987 to the child abuse charge against the still-missing boy and were sentenced to probation, Ojeda said.

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Police were unable to gather enough evidence to charge either of them with the child's disappearance, but the couple did serve a prison sentence for obstruction after lying to detectives "numerous times."

"Shortly after that, Fleming and Luster were released on the obstructing charges and moved to Florida," Ojeda said.

Investigators reopened the case in 2017 after detectives were notified that someone claiming to be Francillion applied for a copy of his birth certificate, Ojeda said. The application turned out to be attempted identity theft.

In reexamining the case, detectives found torn letters between Fleming and Luster while the couple was serving their prison sentence for obstruction that led authorities to believe Fleming may have killed her son.

"Amy wrote in one of the letters, ‘What happened was totally unintentional, I’m sorry, I hope you know that,’” Ojeda said.

Fleming and Luster are still married but have no other children, police said. Investigators do not know how Francillon may have died, but believe child abuse may have played a part in the toddler's alleged murder.

The original detective on the 1986 investigation, retired Lt. Bob King, said he was "overjoyed" to receive a call that an arrest was made in the cold case.

"I've got to say after 32 years, this is one case in my career that I always felt uncomfortable about," King said at the news conference. He said he had been bothered by "the fact that I could not resolve it successfully, and that particularly we’re talking about a 3-year-old little boy."

Authorities have been in contact with Francillon's biological father, John Pierre.

The Clark County District Attorney did not immediately respond to a request for comment from NBC News.

It is unclear whether Fleming has an attorney at this time.