KENNER, La.  The emergency alarm on the Deepwater Horizon was not fully activated the day the oil rig caught fire and exploded, killing 11 people and setting off the massive spill in the Gulf of Mexico, a rig worker on Friday told a government panel investigating the accident.

The worker, Mike Williams, the rig’s chief electronics technician, said the general safety alarm was habitually set to “inhibited” to avoid waking up the crew with late-night sirens and emergency lights.

“They did not want people woke up at 3 a.m. from false alarms,” Mr. Williams told the federal panel of investigators. Consequently, the alarm did not sound during the emergency, leaving workers to relay information through the loudspeaker system.

While it is not known whether it would have saved the workers who died in the April 20 disaster, the lack of a fully functioning alarm hampered the effort to safely evacuate the rig, Mr. Williams said.