NEW ORLEANS - A federal judge is ordering tests to be performed as soon as possible on cement Halliburton Co. used to seal the BP well that later blew out catastrophically in the Gulf of Mexico.

U.S. District Judge Carl Barbier said some of the components may be "deteriorating over time" and that tests should be done "as soon as reasonably practicable."

The cement components had been subpoenaed by federal investigators looking into what caused the April 20 blowout of a BP well being drilled by the Deepwater Horizon rig. Halliburton was hired to seal the well with cement. The explosion aboard the Deepwater Horizon killed 11 workers and led to a spill of more than 170 million gallons of crude in the Gulf.

Halliburton's cementing work on the well has jumped to the forefront of investigations into the explosion. On Thursday, the president's oil spill commission said tests performed before the blowout should have raised doubts about the cement used to seal the well.

The cement mix's failure to prevent oil and gas from entering the well has been identified by BP and others as one of the causes of the accident.

Barbier, who is overseeing lawsuits filed after the April 20 explosion, ordered tests on the same batch of cement used by drillers in the hours before the explosion. Halliburton said it expected government investigators to retrieve the batch of cement components next week, said Cathy Mann, a company spokeswoman. She declined further comment.

Officials with the Coast Guard and U.S. Department of Interior did not return messages seeking comment.

Kenneth E. Arnold, a member of the National Academy of Engineering who served as an adviser to the U.S. Department of Interior during its probes into the Deepwater Horizon explosion, questioned the importance of doing the tests on the cement components.

"The samples are old now," Arnold said. "Whatever tests they do now are going to be open to interpretation." And he added that it would be hard to simulate the cement foam's properties in a laboratory and compare them to what happened inside the well.

He said the more critical piece of the investigation would be to figure out why the engineers and drilling crews on the rig believed the cement job had sealed the well. Before the blowout, crews performed a "negative pressure" test, a procedure that reduces the fluid pressure in the well to ensure there are no gas leaks.

"They interpreted the data to say they had a valid test, which was a misinterpretation," Arnold said. "The real problem here is not whether it was stable or unstable (cement), the test was not proper ... What's critical is to understand why the people on the rig made the decisions they made."

BP acknowledged in a September internal report that its engineers and employees of Transocean Ltd., the company running the rig, misinterpreted the negative pressure test.

But on Thursday, Halliburton came under increased scrutiny when investigators from the president's oil spill commission revealed that tests performed by the company before the deadly blowout showed the cement to be unstable. Independent tests conducted for the commission by Chevron on a nearly identical mixture also concluded that the cement mix was unstable.

In a statement Thursday, Halliburton said it was unable to give the presidential commission the cement mix used on the Deepwater Horizon rig because that batch was being held as evidence for the ongoing federal investigation. Barbier's order released Halliburton to hand over some of the mix to federal investigators.

Specifically, Barbier wants tests on 1 quart of ZoneSeal-2000 and 2 gallons and 1 quart of SCR-100 in Halliburton's possession.

On its website, Halliburton describes SCR-100 as a "cement retarder" that helps make a "uniform slurry consistency from batch to batch." SCR-100 is "synthetically manufactured, which guarantees product uniformity," the company says.

ZoneSeal is the product name for Halliburton's foam cement, a slurry created by injecting nitrogen into cement to secure the bottom of the well. The decision to use foam cement has been criticized by outside experts.

Halliburton said there were "significant differences" between the Chevron tests and those performed by Halliburton. Halliburton said Chevron "tested off-the-shelf cement and additives" while it tested "the unique blend of cement and additives that existed on the rig at the time."

Halliburton says an April test, performed before the explosion, resulted in a stable foam. Halliburton also has pointed blame at BP, contending the company failed to take steps to make sure the cement job worked.

For its part, BP has said a bad cementing job contributed to the blowout. BP has said a "more thorough review and testing by Halliburton" and "stronger quality assurance" by BP's well team might have identified weaknesses in the plan for cementing.

Dave Cohen, a spokesman for the commission, declined to comment on Barbier's order.