Story updated at 2:55 p.m. EST.

The company that drilled BP Exploration and Production's ruptured well in the Gulf of Mexico wants the British oil giant to hand over critical information about the spill and is complaining that the information is being withheld.

Transocean Ltd. attorney Steven Roberts wrote that Transocean has grown frustrated that it has not received key documents even after signing a confidentiality agreement with BP.

"It appears BP is withholding evidence in an attempt to prevent any entity other than BP from investigating the cause of the April 20th incident and resulting spill," Roberts wrote.

Transocean is seeking records of blowout preventer tests, using a new method called Digital Testing, according to the letter. The letter also seeks documents of the Interior Department's approval of the method, which it says is employed only by BP.

The driller is also seeking BP's "shore-based logs," saying that they are needed to confirm transfers of materials and people to Transocean's Deepwater Horizon rig, now sunk a mile below the surface. It is also seeking results of tests done on the well and the cement that was supposed to seal it, along with changes made to BP's "abandonment" plan to temporarily plug the well before production began.

It also wants the tests that Halliburton Co. did on its cement job for the well, located 40 miles offshore.

The information, Roberts wrote, is critical to Transocean's own internal investigation and determining the cause of the fatal explosion and spill. He said that Transocean has promptly responded to BP's requests for documents, but that BP has not produced any documents since June 21, even after Transocean signed the confidentiality agreement. That, he said, contradicts BP's public committment to transparency and fairness.

BP spokeswoman Elizabeth Ashford denied the oil company was withholding information.

"We are disappointed that Transocean has opted to write a letter with so many misguided and misleading assertions, including the assertion that BP is 'withholding evidence' concerning the April 20th accident and the resulting oil spill," Ashford said in an e-mail to Greenwire. "We have been at the forefront of cooperating with various investigations commissioned by the U. S. government and others into the causes of the Deepwater Horizon tragedy. Our commitment to cooperate with these investigations has been and remains unequivocal and steadfast."

BP, Transocean and Halliburton have been pointing fingers of blame at one another almost since the rig sank and the spill began.

At the same time, BP has paid Transocean $1.5 million a day, for months, for its rigs to drill the relief wells intended to permanently seal the well. In addition, Transocean has five other ships working for BP in the Gulf and around the world, bringing in more than $2 million a day (Greenwire, Aug. 11).

Transocean is based in the canton of Zug, Switzerland, where corporate taxes are lower than in the United States. But the letter indicates that Roberts' office is in Houston.

BP's internal investigation into the blowout, explosion and spill is expected to be released sometime this month. The team is led by Mark Bly, BP's head of safety and operations.

The company's preliminary investigation, detailed in May, indicated that a string of mechanical and human errors began when the cement poured into the bottom of the well failed to do its job.

Even once the cement failed, the oil and gas still would not have been able to flood the wellbore if it were not for a series of other missteps and failures, BP said (E&E Daily, May 27).

Numerous other government agencies, commissions and congressional committees are also investigating the spill.

Click here to read the letter from Transocean to BP.