MEXICO CITY, Mexico -­ A buildup of swamp gas under the kitchen of a seaside restaurant at the upscale Grand Riviera Princess hotel was to blame for the deadly explosion that killed seven people Sunday, including five Canadians.

Jesus Puc Pat, the director of civil protection for the Quintana Roo state ­ the district where the hotel is located, in the beach resort of Playa del Carmen - said the gas originated from swampy soil on which the hotel was built.

"The area where the hotel is is very wet and marshy, and such gas originates in the region," he told local media on Monday.

"Preliminary data indicate that the gas was trapped in the space between the floor of the hotel and the caves of the area." QMI Agency has learned that most hotels in the area were built without consideration of the swampy soil beneath, leading to the likelihood that the deadly blast could happen again.

Hotel spokespeople said the area affected by the explosion has been evacuated, but the regular activities are continuing in the rest of the hotel.

Meanwhile, families and friends in Canada are shaken by the deaths of loved ones in the blast.

Five Canadians - including a nine-year-old boy and his father - were among those killed when the portion of the hotel exploded at about 9 a.m. local time.

The blast blew pieces of the floor, glass and chairs out onto the manicured, palm-lined lawns of the sprawling hotel complex.

Officials identified the dead Canadians as: Christopher Paul Charmont, 51, and his nine-year-old son John Paul of Drumheller, Alta.; Malcolm Johnson, 33, of Nanaimo, B.C.; Darlene Ferguson, 52, of Ardrossan, Alta., east of Edmonton; and Elgin Barron of Guelph, Ont.

Two hotel workers, reported to be a tour guide and a security officer, were also among the dead.

Eighteen people were injured, including six Canadians, two seriously, the department of foreign affairs confirmed Monday.

Foreign Affairs Minister Lawrence Cannon said most of the injured are from Ontario.

The foreign affairs department said officials "continue to closely monitor reports of an accidental explosion ... and are providing consular assistance to Canadian citizens as required." Friends and family members looking for information about Canadians they believe are at the Grand Riviera Princess should contact the department's emergency operations centre at 1-800-387-3124 or by e-mailing sos@international.gc.ca.

The hotel is offering therapy to visitors shaken by the events.