NEW ORLEANS — The manslaughter charges brought against two relatively low-ranking BP rig workers in the deadly Gulf of Mexico disaster may be as far as federal prosecutors are willing to go.

The Justice Department has said only that its criminal investigation is still going on.

“Either there simply isn’t evidence that anybody higher up was involved, or the department has concluded the only way it’s going to make its case against more senior corporate officers is if it charges and eventually obtains cooperation” from the two men, said David Uhlmann, a University of Michigan law professor and former chief of the Justice Department’s environmental crimes section.

A federal indictment unsealed last week charged BP rig supervisors Robert Kaluza and Donald Vidrine with botching a crucial safety test before the 2010 drilling-platform explosion that killed 11 workers and triggered the biggest offshore oil spill in U.S. history.

Two and a half years after the blast, they are the only individuals charged directly in the tragedy, despite a string of government investigations that spread fault among a host of people and companies. None of the company’s onshore engineers or executives was accused of wrongdoing in the indictment. BP agreed last week to plead guilty to charges related to the workers’ deaths and pay a record $4.5 billion.