Emergency personnel carry a body of a passenger who died in the TransAsia Airways Flight GE235 plane crash in New Taipei city February 6, 2015. [Photo/Agencies]

TAIPEI -- Rescue workers continued their search along the Keelung River in Taiwan's Taipei for the last three passengers on Sunday, who remain missing four days after the fatal plane crash that has killed at least 40 people onboard.

The three unaccounted passengers are all male, with two tourists from the Chinese mainland and one resident from Kinmen, the destination for TransAsia Airways' flight GE235.

Hundreds of firefighters, policemen and servicemen joined the fifth day searching in an area ranging from 100 meters upstream of the crash site to about 32 kilometers downstream, as well as the maritime areas within a radius of 18 km from the estuary.

An underwater archaeological team with metal detectors also participated in the search on Sunday.

The double-engine turboprop plane, carrying five crew members and 53 passengers, was en route Wednesday to the outlying island county of Kinmen when it crashed shortly after takeoff in Taipei.

Among the 53 passengers, 31 were tourists from the Chinese mainland and the other 22 were residents of Taiwan. One flight attendant and 14 passengers survived.

The exact cause of the crash remains unclear but initial analysis of the flight data recorder and the cockpit voice recorder by the Aviation Safety Council (ASC) showed the right engine of the plane flashed a warning signal just 37 seconds after takeoff and the pilots manually cut the fuel supply to the left engine for unknown reasons, leading to a loss of power in both engines before the plane went down.

The 72-seat aircraft, an ATR 72-600 manufactured by Franco-Italian firm ATR, is able to fly or take off with just one functioning engine, said Yann Torres, of France's aviation accident investigation bureau (BEA) who has joined the investigation.