BOGOTA (Reuters) - Colombian President Juan Manuel Santos on Wednesday called for a thorough investigation of allegations his 2014 re-election campaign may have received $1 million from a Brazilian firm embroiled in a wide-ranging corruption scandal.

Colombia's President Juan Manuel Santos gives his speech after signing a new peace accord with Marxist FARC rebel leader Rodrigo Londono, known as Timochenko, in Bogota, Colombia November 24, 2016. REUTERS/Jaime Saldarriaga

Attorney General Nestor Humberto Martinez has said a portion of the $4.6 million allegedly paid by engineering firm Odebrecht SA to a former senator currently accused of graft may have been funneled to the Santos campaign.

“I ask the electoral commission for a thorough investigation as quickly as possible so all of the truth about the Odebrecht case can come to light,” Santos said on Twitter. Santos’ Cabinet expressed its support in a statement from the interior ministry.

“It’s absurd and unacceptable that people with dubious reputations can, with a simple declaration and without any proof, throw mud at the 2014 presidential campaign,” the statement said.

Martinez admitted on Wednesday that his office has no evidence to back up the allegations made by ex-Liberal Party senator Otto Bula Bula, who was arrested last month on charges of bribery and illicit enrichment.

“For now, the testimony of Bula is the only proof of the entrance of $1 million into the Santos campaign. He has explained the method, time and place of the delivery of that money,” Martinez told reporters. He asked the electoral commission to investigate.

On Tuesday night, Martinez had been more emphatic.

“It has been established that of that amount ($4.6 million), in 2014 Mr. Otto Bula sent two transfers to Colombia, which were cashed at the time, for a total sum of $1 million, and whose final beneficiary was the campaign management of “Santos for President - 2014.”

Santos’ campaign chief and transparency secretary have said the allegations are untrue. The 2014 campaign secured a second four-year term for Santos, who won the 2016 Nobel Peace prize.

Odebrecht’s Colombian office declined to comment. The company is dealing with the fallout from an international bribes-for-contracts scandal unearthed by Brazilian prosecutors.

U.S. prosecutors allege that Odebrecht paid hundreds of millions of dollars in bribes in association with projects in 12 countries, including Brazil, Argentina, Colombia, Mexico and Venezuela, between 2002 and 2016.

Bula has denied allegations he was tasked by Odebrecht with ensuring higher-priced tolls on a highway project.

Santos’s 2014 rival is also facing investigation by electoral authorities for allegedly receiving Odebrecht money. Oscar Ivan Zuluaga was the candidate of the right-wing Democratic Center opposition party.