(Updates death toll, adds details describing the scene, demand for public inquiry)
By Mathieu Belanger
L'ISLE-VERTE, Quebec, Jan 28 (Reuters) - Canadian investigators sifted painstakingly through the charred ruins of a Quebec seniors' residence on Tuesday, seeking clues on what caused a massive blaze last week that authorities fear killed 32 people.
For the first time since fire destroyed much of the wooden three-story building in the small town of L'Isle-Verte in the Eastern Canadian province of Quebec, media were allowed to take pictures of the work of the investigators from a distance.
Police, firefighters and officials from the coroner's office, their breath billowing out in temperatures of minus 20 Celsius (minus 4 Fahrenheit), used rakes, spades and brooms as they looked for evidence.
Some actually sat on the ground in the black debris, raising gentle clouds of ash as they used their hands to examine the remains. One policeman could be seen crouching inside a burned metal door frame that stood in isolation.
Police say around 40 percent of the area has now been cleared of major damage.
In one corner of the site, a special machine that pumps hot air was continuing to melt ice that in some places was 2 feet (60 cm) thick, testament to the amount of water that firefighters used to douse the conflagration.
In other parts of the site blackened furniture, warped bed frames and other wreckage was piled up high.
Police have found 17 bodies and another 15 people remain missing and are presumed dead.
Despite media reports that a cigarette may have ignited the blaze, police say they still do not know the cause and stress it could take them months to work out what happened.
Of the 52 residents in the building, 37 were 85 or older, and some could only move around with the aid of walkers or wheelchairs.
Some residents had made desperate escape attempts, including an old man who tied together bed sheets and tried to climb down from the third floor. The blankets came apart and he fell to the ground but survived.
The Residence du Havre in L'Isle-Verte, a picturesque town about 230 km (143 miles) northeast of Quebec City, was only partially equipped with sprinklers, which are not obligatory at privately run Quebec residences in which some residents are mobile.
Quebec's health minister said on Monday that the provincial government might speed up introduction of tougher laws on installing sprinklers at privately run seniors' residences.
He said the government had not received any complaints about the residence before the fire.
Quebec's association of fire chiefs called on Tuesday for a public inquiry into the disaster, saying all aspects of fire prevention at seniors' residences must be looked at. The provincial government says it is premature to talk about such a probe.
Canadian Prime Minister Stephen Harper will attend a Feb. 1 ceremonial mass in L'Isle-Verte.
The disaster was the second calamity to hit the predominantly French-speaking province in the past year. Last July a crude oil tanker train derailed and exploded in the small Quebec town of Lac-Megantic, killing 47 people. (Writing by David Ljunggren in Ottawa; Editing by Jeffrey Hodgson; and Peter Galloway)