Health officials in Hong Kong are investigating whether the coronavirus may have been transmitted between residents of a high-rise apartment building through bathroom pipes.

CNN reported Wednesday that residents in Hong Kong’s Tsing Yi area were partially evacuated after a 75-year-old man and 62-year-old woman living in the same building became infected with the coronavirus.

Health officials said the two patients live 10 floors apart, raising the possibility that the virus had spread through the building. Officials decided to quarantine dozens of residents living in the building after an unsealed pipe was found in the woman’s apartment.

The woman’s son, his wife and her father were also diagnosed with the virus, according to CNN. Health authorities went on to evacuate residents living in apartments connected to the same pipes.

Hong Kong housing minister Frank Chan said a bathroom pipe in the second patient’s apartment appeared to have been altered.

“A preliminary investigation revealed that the 307-room unit in which the second patient lived may have done self-refitting exhaust pipes,” Chan said.

Researchers are still working to determine exactly how the virus spreads, but the Centers for Disease Control and Prevention (CDC) says person-to-person spreading is thought to occur mainly through respiratory droplets produced when an infected person coughs or sneezes, similar to the flu. It’s not clear if the virus can be spread through fecal matter.

But because the two Hong Kong patients lived near one another in the same building and one apartment was found to have an exposed pipe, authorities are trying to determine whether the coronavirus could have spread through the sewage system.

During the SARS outbreak in 2003, pipes became a source of transmission and it spread through fecal matter. Hundreds of people in Hong Kong died during the SARS outbreak.

Microbiologist Yuen Kwok-yung said Tuesday that an improperly sealed pipe could result in virus transmission by carrying infected feces into the building’s ventilation system, according to CNN.

Hong Kong’s housing minister said the city’s public sewage system is safe, and health experts emphasized that the causes behind the cases in the apartment building remain unclear at this time.