DNA samples have been taken from relatives of a woman held by Colombian rebels to try to determine if her young son is in a Bogota foster home.
Clara Rojas is believed to have given birth in captivity in 2004
Questions over the boy's identity emerged as a Venezuelan-led mission to oversee the release of two female hostages and a child was halted.
Rebels said the release was delayed because of army operations in the area.
But Colombian President Alvaro Uribe said the real reason might be because the rebels did not have the boy.
The Revolutionary Armed Forces of Colombia (Farc) announced last month that they were prepared to release Clara Rojas and her son Emmanuel, as well as a former congresswoman Consuelo Gonzalez de Perdomo, as a goodwill gesture.
Emmanuel, said to be around three years old, is believed to have been fathered by one of Ms Rojas's captors.
Ms Rojas's family are among many relatives anxious for news
Hopes rose at the end of December as Venezuelan President Hugo Chavez, who had been involved in mediation efforts, put together an international mission to oversee the handover.
But on Monday, Mr Chavez read out what he said was a letter from the Farc on Venezuelan television.
The rebels said that continuing operations by the Colombian army had prevented them from turning over the three hostages.
President Uribe accused the guerrillas of lying and said the boy may actually be in a foster home in the Colombian capital.
"The Farc can't keep the promise to free the hostages because they no longer have the child, Emmanuel, in their power," he said.
Colombian authorities say a malnourished boy matching Emanuel's description was brought to a hospital in San Jose del Guaviare in June 2005 and then wound up in a Bogota foster home, his real identity unknown.
Officials said information had recently come to light regarding the boy and who he may actually be.
Hollywood director Oliver Stone was among the international delegation
The government says it will compare DNA from the boy, known as Juan David Gomez, with samples from the relatives of Clara Rojas, who was kidnapped in 2002 along with former presidential candidate Ingrid Betancourt.
"We don't lose anything by doing this," Ms Rojas's brother Ivan said. "Why would we put things in doubt?"
He and his mother had travelled to the Venezuelan capital, Caracas, in the hope that Clara Rojas would be freed.
Whether or not the little boy is Emmanuel will be known in a few days, but according to the BBC's Jeremy McDermott in Colombia this is unlikely to change the plight of hostages in Farc hands.
If it is shown that the Farc lied, they are unlikely to make any concessions and continue with their demands that hundreds of imprisoned rebels be freed in exchange for their hostages.
If the Farc still have Emmanuel, it will give credibility to their accusation and that of President Chavez that the Colombian military sought to block the release.
There are hundreds of hostages held in Colombia
Either way, our correspondent says, the two sides will now trust each other even less, and the hostages will continue in captivity with little prospect of negotiations for their release in sight.
The Farc hold some 45 high-profile hostages, including Ms Rojas, who are considered "exchangeable"
But it is believed the guerrillas hold about 750 hostages in total, some for political leverage but many for ransom.