A key witness failed to corroborate corruption allegations at the heart of the trial of Eni SpA and Royal Dutch Shell Plc over a Nigerian oil deal, potentially bolstering the companies’ defence in one of the industry’s biggest graft cases.

The deal, in which Shell and Eni jointly acquired the rights to an offshore oilfield, Oil Prospecting Licence 245, has spawned legal cases spanning several countries.

Bloomberg reported on Wednesday that the testimony from a high-ranking retired Nigerian police officer, Isaac Eke, at a court in Milan offered no evidence to back up claims from a former Eni manager, Vincenzo Armanna, that the proceeds from the 2011 transaction were distributed as cash bribes.

The failure of what could have been a key prosecution witness helps Shell and Eni, whose Chief Executive Officer, Claudio Descalzi, is among the accused, according to the report.

Both companies and the current and former executives that are also defendants, have consistently denied any wrongdoing.

Shell reiterated that position on Wednesday and had no further comment, according to Bloomberg.

At the centre of the case is the $1.1bn payment Shell and Eni made to the Nigerian government for the OPL 245 in the Gulf of Guinea.

Prosecutors alleged that most of this money was subsequently redistributed as bribes to company and government officials in the country.

Armanna, who is also a defendant in the trial, has alleged that a bodyguard working at the Presidential Villa during President Goodluck Jonathan’s administration, said to be Eke, showed him pictures of trolleys full of cash being taken to a private jet.

Eke told the court he had never set foot in the residence and didn’t meet Armanna until three years after the alleged events took place.

The testimony effectively reversed evidence Eke had previously submitted to the court in a letter.

At one point, the judge interrupted cross-examination to prevent the witness potentially incriminating himself for making false statements to the court.

Eke also testified that a mutual friend told him Armanna would pay for his trip to Milan and the accommodation, but eventually he covered the expenses from his own funds.

The former police officer said he came to the trial unaware that it was a criminal proceeding.

The judge denied a request from Armanna’s lawyer to cross-examine Eke directly.

This is the second time prosecutors have failed to corroborate Armanna’s allegations.

A witness called in December, who was said to be the presidential bodyguard, turned out to be the wrong person. Eke was one of the prosecution’s last two witnesses, and the trial in Milan will now go into its final phase with concluding arguments from both sides before a verdict in the coming months.

Bloomberg reported that the Italian legal system is notoriously slow, with lengthy deliberations and repeated opportunities for appeals after the initial verdict.

It is not infrequent for the statute of limitations to kick in before a final conviction has been reached.