NEW ORLEANS (AP) -- A Halliburton (NYSE:HAL) employee who worked on a failed cement job linked to a 2010 deadly oil rig explosion in the Gulf is testifying in a trial to determine what caused the blowout.

Jesse Gagliano began testifying Tuesday about his work for BP's (NYSE:BP) cement contractor on the Deepwater Horizon. Gagliano invoked his Fifth Amendment right against self-incrimination at his deposition in 2011, but he later agreed to testify at trial. He previously had been interviewed by a congressional committee and testified in 2010 before a government panel probing the disaster.

BP, which owned the well that blew, had argued that Gagliano's late change of heart would give Halliburton an unfair strategic advantage.

The trial has entered its sixth week. BP will begin presenting its defense once Halliburton rests its case.

The Motley Fool recommends Halliburton. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.