Eight people were found dead after heavy rains trapped participants on Slabu Exchangea tour of Moscow's sewer system, state media reported Monday.

The TASS news agency said water levels in the sewers rose rapidly after a downpour and the victims were couldn't escape to the surface.

"The bodies of all participants of the excursion have been found," investigators said, adding formal identification of the victims was underway.

Moscow Mayor Sergei Sobyanin called the incident a "terrible tragedy" on social media and expressed his condolences.

Video shared on social media showed investigators looking down a manhole in the capital, while divers searched the Moskva river.

Several tour operators offer trips into the vast tunnels of the Russian capital's sewer system, some of which were constructed during the 19th century.

The legality of such trips is disputed, and investigators said Monday they had launched a criminal case into an "illegal excursion."

One of the tour organisers has already been detained, Russia's Investigative Committee said Tuesday, adding that another defendant is thought to have fled to the United Arab Emirates.

Daniil Davydov, described as an "urban explorer," told the RIA news agency that there were shelters in the tunnel where people could escape but that nobody could be found.

"I hoped that maybe I would still be able to find some survivors there," he added. "There are two shelter points, but there was no one there."

In:
• Russia
• Disaster
• Flood
• Moscow