Colombian prosecutors say they suspect President Juan Manuel Santos, winner of the 2016 Nobel Peace Prize, received a bribe from scandal-plagued Brazilian construction firm Odebrecht for his 2014 re-election campaign.
Prosecutors said they were still investigating the allegation, which emerged from a probe of a former senator accused of taking and making bribes to ensure Odebrecht won a juicy government contract.
The Santos administration roundly rejected the claim, accusing the right-wing opposition of fabricating it.
The case is the latest spinoff from a giant scandal in Brazil involving the state oil company there, Petrobras, which was bilked for billions of dollars over the course of a decade by corrupt executives, politicians and contractors -- including Odebrecht.
The many officials Odebrecht bribed around Latin America allegedly include Colombian ex-lawmaker Otto Bula.
Colombia's attorney general, Nestor Humberto Martinez, told a press conference that Bula oversaw two transfers to Colombia in 2014 for a total of $1 million, "whose final beneficiary was allegedly the management of the Santos for President 2014 campaign."
Bula was allegedly hired by Odebrecht to help the construction company win a 500-kilometer (300-mile) road project.
Bula, who has been arrested, denies the accusation.
He is close to former president Alvaro Uribe, the fiercest critic of the center-right Santos government.
The administration's transparency secretary, Camilo Enciso, accused Uribe's camp of "defending itself by attacking with lies," after some Uribe-era officials were themselves named in the Odebrecht scandal.
The controversy comes as the Santos government implements a historic peace deal with Colombia's FARC guerrillas and seeks to negotiate another with a rival rebel group, the ELN.
A corruption investigation could damage Santos as he races to end a half-century conflict that has claimed more than 260,000 lives.