There are conflicting theories on what caused a powerful blast that tore through a lounge in a busy Playa del Carmen resort, killing seven people, including five Canadians, over the weekend.
From the start, Mexico's state prosecutor and local officials said trapped gases from a nearby swamp ignited and blew up.
However, on Tuesday, investigators said they had found a ruptured sewer pipe about 10 metres from the site of the blast at the 676-room Grand Riviera Princess resort. They added that it was too early to definitively state the cause of the explosion.
"The investigation is focused on the possibility that leaks and an accumulation of waste water in the foundations could have caused the concentrations of methane, which could have caused this explosion," said Felix Gonzalez Canto, governor of the southeastern state of Quintana Roo.
Olev Trass, a chemical engineering professor at the University of Toronto, told CBC News on Tuesday that the swamp gas scenario was likely.
Swamp gas, also known as marsh gas or landfill gas, is a biogas that's produced when organic material like dead vegetation rots in an oxygen-starved environment such as a swamp, marsh or peat bog.
Canadians killed in Mexico blast
- Darlene Ferguson, 51, of Ardrossan, Alta.
- Malcolm Johnson, 33, of Nanaimo, B.C.
- Chris Charmont, 41, and his nine-year-old son John Charmont, of Drumheller, Alta.
- Elgin Barron of Cambridge, Ont.
"It is fairly likely indeed because it can collect underneath the hotel," Trass said. "As any vegetable matter decays under conditions [where no air is present], they generate methane gas and that has to go somewhere.
"Normally it would go out into the atmosphere and nobody would be bothered by it and you might see the occasional little bubble coming through the water and nothing else. But if you block off that access to the atmosphere, then it builds up and it simply builds up pressure … all you needed was a tiny spark and the whole thing would go up."
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Some resort guests reported a smell of rotten eggs in the days leading up to the explosion. Swamp gas sometimes has a "rotten egg" smell because of the presence of small amounts of hydrogen sulfide produced by the decaying matter. But the most common component of swamp gas is methane, a colourless, odourless and flammable gas.
"Once you get that gas … usually there is some sulphur in the vegetation and, therefore, you get a bit of hydrogen sulphide that has a rotten egg smell and I think that is what can be detected, even in tiny amounts," Trass said.
"I think that smelling that hydrogen sulphide is the best way to find out is there something [wrong] and apparently there was something."
Environmentalists in the area said they had warned authorities not to allow construction on fragile mangrove areas where decomposing organic matter creates methane gas.
However, a delegation from Mexico's Environmental and Natural Resources Secretariat (Semarnat) cast doubts on the swamp gas theory.
Gabriela Lima Laurents, a representative from Semarnat, told reporters on Tuesday that the explosion was likely linked to problems with the operation or maintenance of the hotel.
"I'm 100 per cent sure that it was due to an operational problem in the hotel's infrastructure, and not an accumulation of gas in underground caves," said Lima Laurents.
"If that was the case, not only [the state of] Quintana Roo but the entire [Yucatan] peninsula would be in danger to explode or would have already blown. I believe it has to do with a failing of the hotel, but that will have to be determined through a forensic investigation."
Others have suggested that the wide use of propane and other gases for cooking and heating water in the area might be a more logical explanation for the blast.
In the meantime, the resort remains open to visitors. Trass said people should not be unduly worried.
"I think that I would keep my nose in shape, but otherwise it's such a very rare event and I think that all the newer hotels or buildings built on wetlands in general would have that ventilation system built in."