A BP drilling engineer involved in the planning of the Macondo well declined to testify before a federal investigative panel Friday, invoking through his lawyer his Fifth Amendment right against self-incrimination.

Mark Hafle, who was involved in some of the most heavily scrutinized decisions about the well, became the third BP employee to invoke his constitutional right not to answer questions from the panel. Hafle had testified in an earlier round of hearings.

As Friday's hearing proceeded, another BP employee who wrote one of the most widely derided e-mails to surface in investigations of the Deepwater Horizon disaster testified under oath and gave a more benign explanation of the document.

In an e-mail four days before the April 20 blowout in the Gulf of Mexico, drilling engineer Brett Cocales addressed one of the most controversial decisions in the construction of the Macondo well, saying in part, "who cares, it's done, end of story, will probably be fine."

In the e-mail, he went on to defend the decision based on "the risk/reward equation."

Congressional investigators have cited the decision as evidence that BP might have cut corners to save time and money. Testifying before a separate federal investigative panel in Houston on Friday, Cocales said, "Those are my words."

But he said they had nothing to do with financial considerations.

Cocales said he was weighing engineering risks associated with alternative approaches, and he thought less risk was associated with the course BP took.

At the time, he said, he thought the "worst-case scenario" was that BP would have to do remediation work on the well, not that safety would be jeopardized, he testified Friday.

But he acknowledged that there "was still a risk of channeling" - a term that refers to gaps in the cement lining between the steel well pipe and the rock formation that could give gas a path to escape.

At issue was the number of devices called centralizers installed to center the pipe in the well. Halliburton, a contractor to BP, recommended 21 but BP used six. In a report to BP two days before the explosion, Halliburton warned that with as many as seven centralizers the well could have a "SEVERE" gas flow problem.

Cocales on Friday became the fourth BP employee to testify that, before the blowout, he did not read that warning.