MEXICO CITY (NEXSTAR/WAVY) — Authorities have determined that three Americans who were staying at an Airbnb in Mexico City last month died of gas inhalation.

Police said Tuesday post-mortem examinations suggested Courtez Hall, Jordan Marshall, and Kandace Florence, who were found unresponsive on Oct. 30 in the Airbnb they were renting, died of carbon monoxide poisoning, according to The Associated Press.

Carbon monoxide is commonly found in the fumes from stoves, lanterns, fireplaces, gas ranges, furnaces, grills, or engines, the Centers for Disease Control and Prevention (CDC) explains. Authorities haven’t said what contributed to the tourists’ exposure.

Security guards at the apartment building they were staying in did call police after noticing “an intense smell of gas in an apartment,” the AP reports.

The families of Marshall and Florence told Nexstar’s WAVY earlier this week that the friends had been visiting Mexico City to attend the celebrations surrounding “Dia de Muertos,” or Day of the Dead.

Florence’s boyfriend had been on the phone with her on the night of Oct. 30 when Florence started getting sick, the family said. She told him something wasn’t right. The two eventually were disconnected, and he couldn’t get Florence back on the line. He then called the Airbnb host to do a welfare check, and authorities found Florence, Marshall, and Hall dead.

The victims’ families recounted the difficulties they faced in getting information and their loved ones’ belongings from Mexican authorities.

Marshall’s mother, Jennifer, said they spent hours in a Mexico City police station waiting for a translator to show up. The U.S. embassy was also closed when they arrived, and they were unable to get into the Airbnb to collect her son’s things.

Marshall’s funeral is set for Friday, according to his family.

According to the AP, tourists dying of gas inhalation has been a persistent problem in Mexico. A gas tank explosion in March killed two restaurant employees and injured 18 people at a beachside restaurant on Mexic’s Caribbean coast resort of Playa del Carmen.

Three Americans found dead at a Sandals Resort in the Bahamas in May also died of carbon monoxide poisoning, autopsy and toxicology reports showed. Though authorities haven’t said what exactly caused the exposure, investigators looked into a possible air conditioning or water heater leak.