The explosion killed 11 workers and ultimately caused millions of gallons of oil to leak into the Gulf
The firms drilling a BP Gulf of Mexico oil well had tests showing cement used to seal it before it blew out was unstable, US investigators have found.
The findings conflict with statements by US oil contractor Halliburton, which supplied the cement and has said tests showed it was stable.
But a presidential panel on the disaster found that three tests prior to the blowout showed the opposite.
The 20 April rig explosion killed 11 workers and caused a massive oil leak.
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The blowout on the Deepwater Horizon drilling rig, which was owned by Transcoean and under contract to BP, caused the pollution of hundreds of miles of shoreline and disruption of tourism and fishing before the leaking Macondo well was capped on 15 July.
In a letter to President Barack Obama's national commission on the BP Deepwater Horizon oil spill and offshore drilling, its chief counsel Fred Bartlit said that BP and Halliburton both had test results showing the cement mix used to seal the well before the blowout would be unstable.
Halliburton, which ran the tests, provided some of that data to BP, but investigators found no indication the company had flagged the unfavourable test results to BP or that BP had raised any questions about them.
"Neither acted upon that data," Mr Bartlit wrote.
Halliburton also appears to have kept other test data to itself - one set of results showing once again the cement mix was unstable, and one showing it would hold, investigators found.
The commission's findings go some way to supporting BP's own investigation that found failings in the composition of the cement, says the BBC's Iain MacKenzie in Washington.
However the full report has yet to be published and at this stage the investigators stop short of apportioning blame, our correspondent says.
Bartlit also notes shortcomings by Transocean.
The oil industry has developed tests to identify deficient cement jobs, he wrote.
"BP and/or Transocean personnel misinterpreted or chose not to conduct such tests at the Macondo well," Bartlit wrote.
But Mr Bartlit added that "the story of the blowout does not turn solely on the quality of the Macondo cement job".
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