NEW YORK (MarketWatch) — Halliburton Co. shares slumped nearly 15% Thursday after the presidential commission investigating BP PLC’s blowout in the Gulf of Mexico said the oil-services company may share some of the blame for the disaster.

A letter from the commission’s lead lawyer said the investigation found that Halliburton HAL, -0.16% may have known the cement mix used to seal the Macondo was inadequate for the task.

The well ruptured April 20 as it was being plugged, killing 11 workers, destroying the Deepwater Horizon rig and triggering the worst oil spill in U.S. history.

Markets Hub: Exxon's earnings pack a punch

The commission also pointed out that tests of the sealing job were either misinterpreted or not made by well owner BP BP, -1.18% or the drilling rig’s operator, Transocean Ltd. RIG, -4.26%

Halliburton HAL, -0.16% shares briefly slumped to $28.86. They clawed their way back to end the session with an 8% decline at $31.68.

BP shares rose 50 cents, or 1.3%, to $40.60; Transocean fell 34 cents, or 0.5%, at $64.16.

Halliburton responded in a statement late Thursday saying that it “does not believe that the foam cement design used on the Macondo well was the cause of the incident.”

Halliburton said it is reviewing the government’s report but also raised questions about some of the findings related to tests conducted on the cement before the disaster.

Emails to a BP representative seeking comment were not immediately returned.

The explosion on April 20 came after natural gas seeped out of the Macondo well a mile below the surface of the Gulf of Mexico and into the Deepwater Horizon drilling rig floating on the surface.

At the time, the well had been sealed with drilling mud, cement and other materials in preparation to convert it from exploration to production.

Halliburton and BP both had results in March “showing that a very similar foam-slurry design to the one actually pumped at the Macondo well would be unstable, but neither acted upon that data,” according to a letter to the full commission from Fred Bartlit, the commission’s chief counsel.

The letter, which highlights the panel’s preliminary findings, also said that Halliburton and perhaps BP “should have considered redesigning the foam slurry before pumping it at the Macondo well.”

The letter comes ahead of a two-day public hearing on Nov. 8 and 9 to be held by Bartlit and the commission, with includes Bob Graham and William Reilly as co-chairmen, as well as Frances Beinecke, Donald Boesch, Terry Garcia, Cherry Murray and Fran Ulmer on the roster.

The panel had asked Chevron Corp. CVX, +0.13% to test a similar cement-slurry mixture used on the Macondo well, with Halliburton agreeing that the company’s lab was qualified for the work, Bartlit’s letter said.

“Finally, we want to emphasize that even if our concerns regarding the foam-slurry design at Macondo are well founded, the story of the blowout does not turn solely on the quality of the Macondo cement job,” the letter stated. “Cementing wells is a complex endeavor and industry experts inform us that cementing failures are not uncommon even in the best of circumstances.”

The letter continued to say: “Because it may be anticipated that a particular cement job may be faulty, the oil industry has developed tests, such as the negative-pressure test and cement-evaluation logs, to identify cementing failures. It has also developed methods to remedy deficient cement jobs.

“BP and/or Transocean personnel misinterpreted or chose not to conduct such tests at the Macondo well,” it added.