L'ISLE-VERTE - The official death toll in the Quebec seniors' residence fire stands at 14.

And, Quebec provincial police say 18 people remain missing and are presumed dead in the ruins of the residence in L'Isle Verte.

Police and firefighters are using special machines that pump out hot air at 300 degrees Celsius to melt the thick ice coating the structure.

The work has been slow and painstaking, with workers getting periodic breaks as they brave bitter cold and strong winds along the shore of the St. Lawrence River in eastern Quebec.

Provincial police say they have interviewed about 100 witnesses but no cause of the fire has yet been determined.

A spokesman for Stephen Harper confirmed Monday the prime minister will attend a commemorative ceremony in L'Isle-Verte this Saturday.