The bodies of five people have been found in Moscow’s sewage system and several others are missing after being caught in torrential rain during an underground tour, Russian news agency TASS reported on Monday.

Moscow Mayor Serhiy Sobyanin qualified the event as a “terrible tragedy”, without specifying the number of victims, TASS reports, citing Reuters and News.ro.

The search for the missing continues, investigators are establishing the details of the incident.

TASS said that the authorities have opened a criminal case, and the organizers of the excursion are being interrogated. According to TASS, visiting Moscow’s underground tunnel system would be illegal.

The RIA Agency reports that a total of eight people would participate in this tour.

Heavy rains in Moscow on Sunday caused a sharp rise in the water level in the sewers, which prevented people from getting to the surface, rescuers said.

According to unconfirmed information circulating on social networks, among the dead are an employee of a travel company, his 15-year-old daughter and 17-year-old grandson.

Several guides offer tours of Moscow’s vast sewer system, parts of which date back to the 19th century, Reuters noted.