Police in Mexico City say they believe three U.S. citizens found dead at a rented apartment were victims of gas inhalation.

The city police department said Tuesday the three were found unresponsive Oct. 30 in an upscale neighborhood on the city's west side. They had apparently rented the dwelling for a short visit.

Post-mortem examinations suggested the two men and one woman died of carbon monoxide poisoning. The gas is often produced by improperly vented or leaky gas water heaters and stoves.

Police said security guards at the apartment building called "after they detected an intense smell of gas in an apartment."

Police did not give the victims' hometowns, but local media reported that one was from New Orleans and the other two from Virginia, and were visiting Mexico for the Nov. 1-2 Day of the Dead holiday.

Tourists dying of gas inhalation has been a persistent problem in Mexico, where proper gas line installations, vents and monitoring devices are often lacking.

In March, a gas tank exploded at a beachside restaurant on Mexico's Caribbean coast resort of Playa del Carmen, killing two restaurant employees and injuring 18 people.

In 2018, a gas leak in a water heater caused the deaths of an American couple and their two children in the resort town of Tulum, south of Playa del Carmen.

An inspection revealed that the water heater at the rented condominium was leaking gas. Prosecutors said the gas leak was perhaps caused by a lack of maintenance or the age of the equipment.

In 2010, the explosion of an improperly installed gas line at a hotel in Playa del Carmen killed five Canadian tourists and two Mexicans.

In that case, prosecutors said the gas line, apparently meant to fuel a pool heating unit, was not properly installed or maintained. They said gas leaking from the line may have been ignited in an explosion by a spark from an electric switch or plug.

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