The deaths of three American tourists in Mexico City last month were caused by a gas leak in their Airbnb rental apartment, the prosecutor's office said Wednesday.

The three tourists were found dead in the apartment on October 30 after traveling to Mexico to celebrate the Day of the Dead holiday, American media reported at the time.

"Carboxyhemoglobin was detected in their blood," a source at the prosecutor's office in the Mexican capital told AFP, adding that the cause of death was considered conclusive.

A separate press release from the prosecutor's office said that there had been a gas leak in the tourists' rental apartment.

However, the boyfriend of one of the victims, Kandace Florence, 28, had questioned the official cause of death.

Victor Day told Spanish newspaper El Pais that he had spoken to Florence via video call and she had said she felt "extremely tired" and "as if drugged" after arriving at the property.

Day said he suspects the trio might have been drugged in a bar, and questioned why a gas sensor that the landlord claimed to have was not triggered.

The prosecutor's report said that security guards at the building had called the police after noticing "an intense gas smell in the apartment."

The police found the bodies upon entering.