Several senior U.S. officials said 44-year-old Abu Wa'el Dhiab, a Syrian national, went off the radar several weeks ago from Uruguay, where he was resettled in 2014 after being released.
Uruguayan officials said he traveled to Brazil, but Brazilian officials told CNN they have no record of Dhiab having legally entered their country.
Avianca Brazil Airlines issued an alert warning employees that Dhiab may try to fly using a fake passport, a representative from the airline told CNN. An image of the alert, first published by the Argentine web news portal Infobae, says the information came from Brazil's anti-terrorism police.
Dhiab's disappearance raises concerns about regional security in the weeks leading up to the 2016 Olympics in Rio de Janeiro that are due to begin in August.
A U.S. State Department report on terrorism last year noted that Brazilian police had uncovered a fraudulent document ring in December that had enabled at least 20 Syrian nationals to obtain Brazilian passports.
U.S. officials said Dhiab passed through Brazil in route to Venezuela, where he was last spotted. He is believed to be trying to get to the Middle East, possibly to Syria or Yemen.
"It is not clear whether he wants to go back to the fight or simply go back to the region of the world where he is from and just wants to live there," one senior administration official said, adding that Brazil has the lead on the investigation.
Interpol would not comment on whether Uruguay or Brazil have asked for assistance in tracking Dhiab.
Dhiab was part of a group of six prisoners who were released and transferred to Uruguay in 2014. He was captured by the U.S. military in Afghanistan in 2002 and was accused of being part of an al Qaeda cell, but was never charged and cleared for release in 2009. It took five years for the U.S. to find a country willing to take him.
Uruguay's Interior Ministry told CNN that Dhiab was considered a refugee by the government and, as such, he would not need permission from Uruguayan authorities to leave the country. They said he would need only permission from the foreign country he wished to enter, per an agreement with the U.S. that enabled the release of the Gitmo detainees to Uruguay.
At the time of their transfer, then-President of Uruguay Jose Mujica said on his website: "We have offered our hospitality for humans suffering a heinous kidnapping in Guantanamo. The unavoidable reason is humanitarian."
Mujica also said that his government would place no restriction on their movements, adding that "the first day they want to leave, they can go," according to a report in The New York Times.
But several senior administration officials have told CNN that under the agreement, Uruguayan authorities agreed to keep track of former detainees resettled in their country.
"They should be careful calling this guy a refugee because he is not one," one senior State Department official told CNN, adding, "Under the agreement they agreed to keep an eye on him to make sure he didn't return to the fight."
U.S. officials have noted a perceived lack of political will on the part of the Uruguayan government to keep tabs of the detainees or do anything on their behalf. The U.S. government is in the process of re-engaging with the Uruguayans to ensure they meet their responsibilities to monitor and provide for the detainees, the officials said.
Dhiab and the other detainees have been very vocal about their unhappiness in their new homes in Uruguay, saying they felt alienated in a country where they did not speak the language and there is no Muslim community. They have complained that the government had not been doing enough to help them get jobs and that they continued to be separated from their families.
Dhiab's disappearance could provide fuel for opponents of efforts to close the detention facility at Guantanamo, especially if Dhiab is found to be attempting to join a terrorist group. Of the 676 detainees released from the detention facility as of January, 118 have returned to the fight and an additional 86 are suspected of returning, a recidivism rate of nearly 1 out every 3 released, according to the most recent report
from the Office of the Director of National Intelligence.