The emergency location transmitter (ELT) of the ill-fated light plane carrying the late Interior and Local Government Secretary Jesse Robredo failed to activate when it crashed off the central Philippine province of Masbate on Saturday afternoon, an official said on Friday.

Civil Aviation Authority of the Philippines (CAAP) director general William Hotchkiss III said this was the result of the initial inspection conducted by the three-man Aircraft Accident Investigation and Inquiry Board (AAIIB) investigating team.

CAAP noted that an ELT is a plane device that automatically activates when a plane encounters emergency landing or any disasters. It transmits a distress signal to make it easier to find an aircraft which crashed.

Hotchkiss said the CAAP last checked the Piper Seneca plane's ELT on November 21, 2011 and is valid for operation within a year.

"It is too early to speculate. Our investigating team is still examining the plane wreckage to determine the cause of the accident," he said in a statement.

The ELT was found with the plane wreckage last Wednesday but the right engine of the aircraft has yet to be recovered, he said, adding that the wreckage is now under tight security at Masbate airport.

Hotchkiss said search and retrieval of the plane engine is ongoing.

"The engine is a key part of our investigation. This will determine if indeed it was due to technical or mechanical problem that caused it to fail," he said.

Aside from Robredo, two pilots also died in the plane crash. Only the aide of the late official survived.