BOGOTA (Reuters) - A former Colombian congressman detained on accusations he received millions in bribes from Brazilian engineering firm Odebrecht SA [ODBES.UL] denied on Tuesday that a $1 million portion of the money was given to President Juan Manuel Santos’ 2014 campaign.

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. Picture Taken November 24, 2016. REUTERS/Jaime Saldarriaga

Attorney General Nestor Humberto Martinez said last week that part of $4.6 million allegedly paid to ex-Liberal Party Senator Otto Bula Bula by Odebrecht was sent to Santos’ campaign management.

Martinez later said his office had only the testimony of Bula, who was arrested last month on charges of bribery and illicit enrichment, to substantiate the allegations.

But Bula said via a hand-written letter released by the electoral commission, which is investigating the allegations, that he had not given money to the campaign.

“It is not true, it is not a fact, I have not said that the money I turned over ... was a contribution to the Santos presidential campaign,” the letter said.

Bula’s testimony will likely end the commission’s investigation into the campaign, which secured a second four-year term for Santos, judicial sources said.

The campaign of Santos’ right-wing rival Oscar Ivan Zuluaga is also under investigation by the commission for allegedly receiving $2 million from Odebrecht. Zuluaga was the candidate of the Democratic Center opposition party.

Parties may be disbanded and prevented from backing candidates if a campaign is found guilty of electoral violations.

Odebrecht pleaded guilty in U.S. court and agreed to pay a $3.5 billion penalty for paying hundreds of millions of dollars in bribes in association with projects in 12 countries, including Brazil, Argentina, Colombia, Mexico and Venezuela.

Bula is accused of working to ensure higher-priced tolls on a highway project on behalf of Odebrecht in exchange for bribes.

Gabriel Garcia Morales, a former vice minister of transport, has also been arrested for allegedly receiving $6.5 million in bribes from Odebrecht in 2009.