BP PLC had problems with its cement contractor years before a cement failure contributed to last spring's massive oil spill in the Gulf of Mexico, a chief investigator for the presidential panel investigating the blowout said today.

Fred Bartlit, the chief counsel for the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, wrote in a report out today that BP knew of and had experienced several problems with Halliburton Co.'s work in the years leading up to the disaster.

The commission, in its final report to the president last month, criticized Halliburton's cement job on the project, saying the company may have completed the work without knowing whether its cement was stable. And Bartlit's new report says BP was aware of problems with Halliburton's work years before the disaster.

Bartlit cites a 2007 independent quality control report that warned the company that Halliburton's lab technicians were not experienced in evaluating data and BP needed to improve its communication with the contractor "to avoid unnecessary delays or errors in the slurry design testing."

BP's engineers and experts also cited their own concerns with Halliburton's work, Bartlit said. For instance, he reports that BP's cementing expert described Halliburton's work as "operationally competent and just good enough technically to get by."

But BP continued working with Halliburton despite the concerns, Bartlit reports.

BP's engineers on the ill-fated Macondo well had complaints for years about the Halliburton engineer assigned to the project, saying he was "not cutting it" and that he waited too long to conduct crucial tests. But despite the concerns, they did not review his work or check to see that he conducted tests on time, Bartlit wrote.

"The sad fact is that this was an entirely preventable disaster," Bartlit said in a statement.

Bartlit's report follows the release of the commission's final report last month. Today's document expands on the causes of the disaster that killed 11 rig workers and sparked the largest oil spill in U.S. history and provides new details about the events leading up to the disaster.

Another key finding in Bartlit's report is that the failures of the well's blowout preventer were not the root cause of the blowout.

Republican lawmakers criticized the commission last month when it released its report, saying the panel should have waited to make its assessments until after a forensic examination of the blowout preventer were completed.

The rig crew activated the [blowout preventer], at best, only moments before the blowout began. By then, hydrocarbons had already gone past the [blowout preventer] into the riser and were expanding rapidly towards the rig floor," the report says. "Even if the [blowout preventer] had functioned flawlessly, the rig would have exploded and 11 men would have died."

The report also says that BP had experts on the drilling rig during crucial tests that could have prevented the sequence of events that led to the disaster. But the experts were not consulted, the report says.

"If anyone had consulted him or any other shore-based engineer, the blowout might never have happened," the report says.

Spokesmen for BP and Halliburton did not respond to requests for comment.

Separately, three workers who helped BP in its spill cleanup efforts last summer are accusing the company today of denying them overtime pay and other benefits.

In a lawsuit filed today in U.S. District Court in New Orleans, the workers say BP and its cleanup contractor misclassified them as independent contractors and denied them overtime pay for their work.