A massive blast that killed five Canadian tourists and two Mexicans at a luxury beach resort appears to have been caused by gas from a nearby swamp, local authorities suggested Monday.

Nine of 18 people hurt in Sunday's blast in the Grand Riviera Princess hotel in Playa del Carmen, south of Cancun in Quintana Roo state, were released from hospital Monday.

Preliminary investigations indicated "an accumulation of gas caused by the decomposition of organic material" underneath the building was behind the explosion, which blew out windows, said state attorney general Francisco Alor.

The damaged area of the hotel had been built four years ago on top of a swampy area, Alor said on Milenio television, ruling out that the blast was caused by an attack.

However, an environment ministry representative warned against making "bold statements."

"It's unlikely that there was an accumulation of gases," Gabriela Lima, a state official from the environment ministry, told AFP, calling on local authorities to wait for a full investigation.

Lima said that had it been due to gases, there would be more frequent explosions in the area given that many buildings sit on top of mangroves and holes in the ground.

Canadian officials confirmed the deaths of five Canadians and said that six others were hurt.

They were working with bereaved families, tour operators and authorities to return the bodies as soon as possible to Canada, a statement said.

"We have nine more in hospital, all in a stable condition," including four Canadians, two US citizens and three Mexicans, Alor said.

The deceased Canadians were three men, one woman and a nine-year-old boy, and the Mexicans were a tourist guide and the hotel's security director, Alor said.

Photographs showed collapsed ceiling panels surrounded by piles of rubble, next to a beach lined by palm trees.

Playa del Carmen is around 90 kilometers (56 miles) south of the major resort of Cancun, in an area known as the Maya Riviera.

Mexico's Caribbean coast hosts about two million tourists every year, mostly from the United States, Canada and Europe.

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