HOUSTON  An official for Halliburton, the company hired by BP to perform a critical step in the process for closing the well connected to the Deepwater Horizon oil rig, testified Tuesday that days before the rig exploded he had raised concerns to BP about its plan for executing the procedure, but that he continued with the job anyway.

The official, Jesse M. Gagliano, a shore-based technical adviser, told federal investigators here that he had recommended that BP use a greater number of devices called “centralizers” in the well for the tricky step known as cementing, which is a method of strengthening the well to control pressure from the oil and gas. He said he was ignored.

Centralizers help cement flow evenly around an oil well before hardening, and using 6 of them  instead of the 21 that he recommended  made the well more likely to need additional cementing, Mr. Gagliano said.

Investigators say they believe that the cement poured by Halliburton may have failed under tremendous pressure on April 20, producing the oil rig explosion that led to the largest deep-water oil spill in United States history.