NEW ORLEANS — Federal authorities have blamed two rig supervisors for the deaths of 11 workers killed when the Deepwater Horizon exploded and spawned BP’s massive oil spill in the Gulf of Mexico, even though a string of investigations spread fault among a host of people and companies.

A federal indictment last week charged Robert Kaluza and Donald Vidrine with manslaughter, accusing them of botching a crucial safety test. But a defense attorney claims the two BP well site leaders are scapegoats, given that other government probes have concluded the disaster resulted from a complex web of mistakes and a corporate culture that placed profits ahead of safety.

One expert speculated the Justice Department probably took aim at targets higher up BP’s corporate ladder before charging Kaluza and Vidrine.

“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 Vidrine and Kaluza,” said David Uhlmann, a University of Michigan law professor and former chief of the Justice Department’s environmental crimes section.

Although London-based BP PLC has agreed to plead guilty to charges related to the workers’ deaths, none of the company’s onshore engineers or executives is accused of wrongdoing in the indictment unsealed last Thursday. Former BP executive David Rainey was charged separately with withholding information about the spill from Congress, but the allegations against him aren’t related to the causes of the blowout.

Shaun Clarke, one of Kaluza’s attorneys, said the narrow focus of his client’s indictment doesn’t jibe with the widely accepted conclusion that “multiple failures at multiple levels in multiple companies” led to the blowout.

“It would have taken a lot of courage after spending three years and tens of millions of dollars investigating to go back to the White House and say, ‘You know, Mr. President, we can’t really find a person to blame.’ Instead, they decided to scapegoat two people who were just out on the rig doing their jobs,” Clarke said.

Natural Resources Defense Council president Frances Beinecke, who served on a presidential commission charged with investigating the explosion and subsequent spill, said the disaster resulted from “systemic failures” that raised concerns about the entire drilling industry’s safety culture.

“You can fault two people for this accident, but you cannot leave it there,” she said. “You have to look at what’s happening throughout this industry.”

That commission concluded the blowout was the product of “several individual missteps and oversights” by BP, rig owner Transocean Ltd. and cement contractor Halliburton, with mistakes made both on the rig and onshore. The panel also found that government regulators lacked the authority, resources and technical expertise to prevent the mistakes from occurring.

“Though it is tempting to single out one crucial misstep or point the finger at one bad actor as the cause of the Deepwater Horizon explosion, any such explanation provides a dangerously incomplete picture of what happened — encouraging the very kind of complacency that led to the accident in the first place,” the commission’s January 2011 report says.

As well site leaders, Kaluza and Vidrine were the highest-ranking BP supervisors on the rig, each having four decades of experience in the oil patch.

Vidrine, 65, was on duty at the time of the April 20, 2010, explosion. Kaluza, 62, was filling in for another well site leader and had been on the rig only a few days before the blowout. He was in bed at the time of the blast.

The case against Kaluza and Vidrine centers on their roles in supervising “negative testing,” which is designed to assess whether a cement barrier is effectively preventing oil or gas from flowing up the well.

The indictment says they had “multiple indications” from the negative testing that the well wasn’t secure. Yet they allegedly failed to alert onshore engineers about the problems during the testing, accepted a “nonsensical explanation” for abnormal pressure readings and eventually decided to stop investigating.

When another rig worker attributed the pressure readings to a so-called “bladder effect,” Vidrine and Kaluza accepted that explanation.

“This explanation was scientifically illogical and was not recognized within the deepwater oil exploration industry,” the indictment says.

Uhlmann said it’s understandable that prosecutors would focus on the well site leaders’ conduct because they were in charge of rig operations. However, he said the indictment could send a troubling message that the failures were limited to actions on the rig and not at higher levels of the company.

“Corporate culture and management policies are created far above the well site leaders,” he said.