"Halliburton (and perhaps BP) should have considered redesigning the foam slurry before pumping it at the Macondo well, " the letter continued.

CNN: STORY HIGHLIGHTS

"We have known for some time that the cement used to secure the production casing and isolate the hydrocarbon zone at the bottom of the Macondo well must have failed in some manner, " the letter said, noting that BP used a "nitrogen foam cement" recommended and supplied by Halliburton.

CNN: STORY HIGHLIGHTS