Former Sen. Piedad Cordoba, the go-between in all 18 releases by the Revolutionary Armed Forces of Colombia, or FARC, since early 2008, did not offer an explanation except to say via Twitter that she was "certain we will soon see them freed."
She wrote from Ibague, a provincial capital in Colombia's central cordillera, the staging area for Sunday's missions. The Red Cross also did not elaborate.
The government's delegate, Eduardo Pizarro, told reporters that after picking up police patrolman Carlos Ocampo, a 30-year-old held since December, the loaned Brazilian military helicopter emblazoned with Red Cross logos went to a different jungle location whose coordinates had been provided to Cordoba by the FARC.
Pizarro said "the FARC has committed a scandalous act."
He said the government would evaluate the situation with the Red Cross and other involved parties on Monday. He said it "is committed to the liberation" of police Maj. Guillermo Solorzano, 35, who was captured in June 2007, and of army Cpl. 1st Class Salin Antonio San Miguel Valderrana, 27, captive since May 2008.
The FARC now holds fewer than 20 police and soldiers, whose freedom it has proposed exchanging for that of jailed rebels.
Badly battered by government air raids and record desertions during President Juan Manuel Santos' 2006-2009 tenure as defense minister, the Western Hemisphere's last remaining rebel army is also seeking a dialogue that might open a path to peace talks.
Santos on Saturday criticized what he termed the "media show" surrounding the FARC rebels' piecemeal liberations this week.
The first in nearly a year, they began on Wednesday with the release of a town councilman and continued Friday when the guerrillas freed another councilman and a young soldier. Santos called the releases a "farce" because the FARC continues to kidnap people and won't cease hostilities.
In an interview published Sunday by with Semana magazine, the president also said that "no one in this government is authorized to make any contact of any kind with the FARC."
Analysts have suggested that if any contact were made it would be secret and through third parties.