Colombia's government and the US have denied that President Alvaro Uribe collaborated with a powerful drugs cartel more than a decade ago.
Uribe has trumpeted his war against the cocaine trade
Recently declassified US military intelligence from 1991 appears to show that Mr Uribe had links to the notorious Medellin drugs cartel.
The unnamed source even describes Mr Uribe as a "close personal friend" of late cartel kingpin Pablo Escobar.
But Bogota dismissed the allegations as an attempt to smear the president.
'Dedicated to collaboration'
The declassified document was released by the US Defense Intelligence Agency after a request from the National Security Archive - a non-governmental research group - under the Freedom of Information Act. It is now available to view on the NSA website.
Dated 23 September 1991, the document is a numbered list of "the more important Colombian narco-traffickers contracted by the Colombian narcotic cartels".
At number 82 is: "Alvaro Uribe Velez - a Colombian politician and senator dedicated to collaboration with the Medellin cartel at high government levels.
"Uribe was linked to a business involved in narcotics activities in the US... Uribe has worked for the Medellin cartel and is a close personal friend of Pablo Escobar Gaviria."
The intelligence source also claims Mr Uribe's father was killed because of his son's links to drugs traffickers and that Mr Uribe opposed a US-Colombia extradition treaty.
The NSA concedes that the reliability of the intelligence is difficult to weigh because the source's identity has been withheld.
But the group also points out that much of the information given can be easily verified as accurate, and that the document contains specific details, giving the impression it has been carefully researched and compiled.
In a statement, Mr Uribe's office insisted his father was killed by left-wing Farc guerrillas for resisting kidnap. It points out that Mr Uribe has authorised a record 170 extraditions to various countries as president.
It also says Mr Uribe "has not had business of any kind outside of Colombia".
But the statement did not specifically rebut allegations that Mr Uribe was a close friend of Escobar or had links to the Medellin cartel.
Mr Uribe, who celebrates two years in the presidency this week, is now viewed as a close US ally.
In Washington, a spokesman for the state department said it "completely disavowed" the allegations.
Uribe is alleged to have been a "close friend" of Pablo Escobar
"President Uribe's record is one of a strong opponent of drug trafficking," said Robert Zimmerman, deputy spokesman on Western Hemisphere affairs.
The Colombian government was criticised last week for granting three warlords from the country's most notorious paramilitary group, the AUC, immunity from arrest to address Congress.
The president's policy to crush left-wing rebels - which includes mass round-ups of suspected rebel collaborators and lenient sentences for paramilitaries who renounce trafficking and violence - has been criticised by human rights groups.
Despite that, after four decades of devastating civil war, many Colombians are pleased with Mr Uribe's hardline stance against the guerrillas.
Mr Uribe's office pointed out that similar allegations were made against Mr Uribe in the run-up to his election in 2002.
Colombians elected him in a landslide victory despite those allegations.