Transocean, owner and operator of the drilling rig that caught fire and sank in the Gulf of Mexico last year, has concluded that the oil well’s owner, BP, was primarily responsible for the massive spill triggered by the blast.

In a report issued Wednesday after an internal investigation, Transocean said BP used a “faulty” well design, failed to check the integrity of cement used to seal the well and chose a plan for temporarily abandoning the well that “created unnecessary risk.”

Transocean acknowledged the possibility that its crew members failed to detect the signs that hydrocarbons were breaching the cement and steel barriers in the Macondo well and making their way up the drill pipe. Those signs should have raised alarms about a possible blowout, its investigators found.

“None of the individuals monitoring the well, including the Transocean drill crew, initially detected the influx,” the report said, noting that “the results of the critical negative pressure test were misinterpreted.”

But in a conference call Wednesday morning, Bill Ambrose, Transocean’s manager for North America, said that “bad information” BP gave to the rig crew and other rig operations “masked” the signs of trouble. He said it remains uncertain what key Transocean workers saw or thought before the disaster because they died in the accident.

Eleven people on the Deepwater Horizon rig were killed in the blowout, and oil spilled into the gulf for nearly three months.

BP dismissed the Transocean report as “an advocacy piece, in which Transocean has cherry-picked the facts in support of its litigation strategy.”

Transocean’s report says that BP came up with “at least five different temporary abandonment plans” for sealing the Macondo well from April 12 to 20 last year, before choosing one that Transocean said “did not have the required approval” of the Interior Department’s Minerals Management Service.

It also says that, given the difficult conditions of the well and earlier problems controlling the oil and gas in the reservoir, BP’s well design — known as long-string casing — “would increase the risk of exceeding the margin for safe drilling.”

In its statement Wednesday, BP said that investigations by a presidential oil spill commission and the U.S. Coast Guard had concluded that the Deepwater Horizon accident was “the result of multiple causes, involving multiple parties, including Transocean.” The oil giant noted that the Coast Guard cited “serious safety management system failures and a poor safety culture” at Transocean.

BP has questioned why the blowout preventer, owned by Transocean, did not cut off the drill pipe and seal the well once oil and gas started surging up the well. But the Transocean report says that the Deepwater Horizon’s blowout preventer “was properly maintained and operated.”

The question of responsibility for the blowout on the Macondo well is a multibillion-dollar one. BP has reserved more than $40 billion to cover cleanup costs and legal claims. On May 20, a unit of Mitsui, which owned a 10 percent stake in the well, agreed to pay BP $1.1 billion to settle claims. On June 20, Weatherford, an oil-well-equipment firm that made a device known as a float collar used in the Macondo well, agreed to pay BP $75 million.