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The attorney-general's office in Colombia has opened an investigation into the death of Jorge Enrique Pizano.

Mr Pizano was a key witness in a corruption investigation involving Brazilian construction giant Odebrecht.

He died on Thursday of what forensic experts originally described as a heart attack.

But investigators became suspicious when Mr Pizano's son died three days later of poisoning after drinking from a bottle left at his father's home.

Alejandro Pizano had travelled from Barcelona, where he works as an architect, to Colombia to attend his father's funeral.

While at the family home north of the capital, Bogotá, he took a swig from a bottle of flavoured water which was standing on his father's desk.

Witnesses said he immediately became violently ill and died on the way to hospital. Doctors diagnosed cyanide poisoning.

His poisoning has led investigators to re-examine the forensic tests carried out on his father's body.

The possibility that Jorge Enrique Pizano's death may not have been due to natural causes has caused a stir in Colombia with many speculating about who could have wanted to silence the engineer.

Mr Pizano worked as an auditor for the Ruta del Sol (Sun Route), one of Colombia's biggest infrastructure projects aimed at constructing roads to link up the centre of the country with the Caribbean coast.

The contract to build much of the 1,000km (620 miles) stretch of motorway was awarded to Brazilian construction giant Odebrecht and its local partners.

But Odebrecht's concession was cancelled in February 2017 after evidence emerged suggesting the company had paid bribes to be awarded the contract.

Odebrecht is at the centre of a massive corruption scandal spanning Latin America and, as part of a plea deal with the US justice department, it has admitted paying bribes in 12 countries, including Colombia.

Prosecutors in Colombia estimate the firm paid $27m (£20.9m) to land the contract for the Ruta del Sol project.

Mr Pizano was hired as an auditor in 2010 and in 2013 began denouncing what he said were suspicious payments made by Odebrecht and Grupo Aval, Colombia's largest banking group which was also involved in the Ruta del Sol project.

In August, Mr Pizano handed Colombian news programme Noticias Uno a secret audio recording he said he had made in 2015 of himself flagging up the payments to Néstor Humberto Martínez. Mr Martínez was Grupo Aval's legal adviser at the time and is now Colombia's attorney-general.

Noticias Uno says Mr Pizano feared there were people plotting against him and handed their reporter the recording and other documents for safekeeping either until he was safe in another country or for publication in the case of his death.

Noticias Uno broadcast the audio on Sunday night and it has triggered calls for Mr Martínez to resign.

Mr Martínez said in a statement that Mr Pizano had told him about irregular payments at the time but that it was not clear that those payments were bribes. Mr Martínez also said that he had passed on Mr Pizano's concerns to company officials at the time.

Grupo Aval has released a statement saying it had "immediately ordered an internal audit into Dr Pizano's suspicions" at the time and that it had not been aware of any bribes being paid before 21 December 2016, when Odebrecht admitted to the payments as part of the plea agreement.