* Alarm could have alerted crew to dangerous gas buildup

* Some key rig witnesses decline to appear before panel

HOUSTON, July 23 (Reuters) - An emergency alarm that could have warned workers aboard the doomed Deepwater Horizon Gulf of Mexico drilling rig was intentionally disabled, a rig engineer told U.S. investigators on Friday.

Mike Williams, chief engineer technician aboard Swiss-based Transocean Ltd's RIGN.SRIG.N rig, said the general alarm that could have detected the cloud of flammable methane gas that enveloped the rig's deck on April 20 was "inhibited."

“They (rig managers) did not want people woke up at three o’clock in the morning from false alarms,” Williams told a six-member federal board in the New Orleans suburb of Kenner, Louisiana.

Williams’ testimony capped a week of testimony from company officials involved in the rig, which exploded on April 20 and sank two days later, killing 11 crewmen and sparking the worst oil spill in U.S. history.

The Transocean-owned rig was drilling a well a mile (1.6 km) beneath the Gulf under contract for London-based BP Plc. BP.LBP.N.

Four Transocean witnesses declined to appear voluntarily on Wednesday at the hearings before a joint U.S. panel convened by the U.S. Coast Guard and the Interior Department’s Bureau of Ocean Energy Management, Regulation, and Enforcement.

The board on Thursday declared two BP officials “parties of interest” in the investigation, after they declined to appear.

The panel will convene for another week of hearings in Houston from Aug. 23 to 27, where high-level managers from BP and Transocean are scheduled to testify.

At this week’s hearings, Transocean officials recited a litany of mechanical problems that plagued the rig, which was 43 days behind schedule in drilling the Macondo well, which rig workers referred to as the “well from hell.”

Williams, who has filed a lawsuit against Transocean, said a computer system that monitored well drilling operations, known as the “A chair,” was often offline due to technical issues.

“We called it the blue screen of death,” Williams said. “It would turn blue and you would have no data coming through.” (Reporting by Chris Baltimore; Editing by Peter Cooney)