The 5th U.S. Circuit Court of Appeals upheld a lower court ruling that granted Kurt Mix a new trial because of jury misconduct in his 2013 trial.
Prosecutors accused Mix of deleting text messages about the amount of oil flowing from BP's Macondo well after the 2010 Deepwater Horizon disaster. He was acquitted on one criminal count at his 2013 trial but convicted on one count of obstruction of justice.
However, U.S. District Judge Stanwood Duval later ruled that the jury forewoman had tainted deliberations.
According to the court record, she had overheard on a courthouse elevator that other BP employees were being prosecuted in addition to Mix. She later told deadlocked fellow jurors that she had heard something that, in the words of Tuesday's ruling, "increased her confidence in voting guilty."
Mix has pleaded not guilty. His defence team says there is ample evidence he shared information about the flow rate throughout the government investigation. They also said prosecutors failed to prove Mix knew the information he deleted would be pertinent to a grand jury investigation — an investigation they said he did not know about and that had not yet even begun.
In upholding Duval's decision, a three-judge 5th Circuit panel said federal prosecutors failed to prove that the jury forewoman's remarks were harmless and had not prejudiced the jury.
"The government now argues that the evidence against Mix was so overwhelming that the extrinsic information was irrelevant to his conviction," Judge Edith Brown Clement wrote on behalf of the panel. "This argument fails."
A new trial date has not yet been set.
The Justice Department declined comment on the 5th Circuit ruling through spokesman Peter Carr.
"We appreciate the care with which the Fifth Circuit reviewed the facts and understood the trial record, and are gratified by its decision," defence attorney Joan McPhee wrote in an emailed statement.
Tuesday's ruling came weeks after a federal jury acquitted David Rainey, a former vice-president for BP Exploration and Production Co., on charges that he lied to investigators looking into the spill.
Trial is pending for BP well site leaders Robert Kaluza and Donald Vidrine, who have pleaded not guilty to manslaughter charges stemming from the deaths of 11 workers who were on the Deepwater Horizon when it exploded.