A drilling company supervisor stationed on the Deepwater Horizon rig before its April 20 explosion participated in a government-commissioned investigation of the blowout preventer that failed to stop the oil spill - a possible conflict of interest that a congressional critic says may threaten the probe's integrity.

According to documents obtained by Hearst Newspapers, the Transocean Ltd. employee went so far as to manipulate equipment on the hulking 60-foot-tall, 300-ton device while a firm under contract to the U.S. government ran it through a battery of tests in New Orleans.

Transocean owned the Deepwater Horizon rig - which included the blowout preventer - and BP leased it to drill the doomed Macondo well.

Because the examination is essential to learning why the blowout preventer didn't work as planned, the Transocean employee's involvement raises "serious questions as to the credibility and objectivity of (the government's) investigation," said Rep. Ed Markey, D-Mass., the head of a House Energy and Commerce subcommittee that has taken a lead role in probing the disaster.

"If we are to hold the companies legally responsible for this accident, we can't afford any black mark on the investigation involving the 'black box' of this underwater disaster," Markey said in a letter Tuesday to Michael Bromwich, director of the Bureau of Ocean Energy Management, Regulation and Enforcement.

The bureau and U.S. Coast Guard are jointly investigating the Deepwater Horizon disaster and are overseeing the autopsy of the blowout preventer, which began in mid-November. The government contracted the forensic analysis firm Det Norske Veritas to run the equipment through tests designed to shed light on why key pipe-cutting and hole-closing components failed to slash through drill pipe and seal off the well hole.

The analysis firm later arranged with the onetime subsea supervisor on the Deepwater Horizon, Owen McWhorter, to assist in the testing. But after concerns were raised last week by the Chemical Safety Board, a federal agency that also is investigating the Deepwater Horizon disaster, the government instructed the firm to terminate its contract with McWhorter.

Board Chairman Rafael Moure-Eraso had noted that "McWhorter was not on the rig at the exact time of the incident" but stressed that he "still had responsibility for the (blowout preventer) in the preceding weeks and months."

Photos of the testing space obtained by Hearst Newspapers show a man identified as McWhorter manipulating a pipe ram on the blowout preventer, working to remove drill pipe from it and touching other equipment on the device over at least a two-week period beginning as early as Nov. 23.

Attempts to contact McWhorter were not successful.

Access to the testing space is limited to workers for the analysis firm and one representative each from key parties, including BP, the well owner; Cameron International, which manufactured the blowout preventer; and Transocean.