Colom was detained on corruption charges along with nine other people, including Juan Alberto Fuentes Knight, chairman of Oxfam International.
Oxfam International Executive Director Winnie Byanyima said the aid organization does not know the nature of the formal charges against Fuentes.
"However, he has been entirely open with his Oxfam board and executive that he has been among former officials being investigated as part of a budgetary transaction made by the Guatemalan government while he was a finance minister," Byanyima said in a statement.
Fuentes assured Oxfam that he has cooperated fully with the investigation in the confidence he did not knowingly transgress rules or procedures, Byanyima said.
Fuentes told CNN's Michelle Mendoza on Tuesday that he was one of the signatories for the agreement with the company that operated the transportation service. But he said that when he noticed irregularities he asked for the contract to be investigated.