Eight people are feared dead after heavy rains washed away a Moscow sewer tour group. AnnaArinova

Multiple people are dead after floodwaters washed away a Moscow sewer tour group.

Russian state media reported that torrential rains left an illegal tour group stuck underground.

Officials are still searching the Moskva river for bodies.

At least five people are dead after heavy rains flooded Moscow's sewer system, washing away a tour group of eight people who were exploring the historic underground system.

Moscow Mayor Sergey Sobyanin confirmed the accident in a Monday Telegram post, saying law enforcement was investigating the details of the doomed "self-organized descent."

Russian state media outlet Tass News Agency reported a teenage girl was pulled from the river on Sunday night following the flooding. Two more tour members, a man and a teenage boy, were found in the river early Monday morning, the outlet said.

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The fourth and fifth victims were also discovered in the river Monday afternoon, the outlet reported, citing an unnamed emergency official.

Insider could not independently verify the number of confirmed dead.

Authorities are still looking for the remaining missing as divers search the Moskva River for more bodies.

Photos and videos posted to social media appeared to show officials looking down a manhole cover, as well as investigators searching the river by boat.

Torrential downpours walloped Moscow on Sunday, causing a rapid rise in water levels inside the underground tunnels. The rains accounted for about a third of Moscow's monthly rainfall, Russian state media outlet Ria Novosti reported.

The heavy rains resulted in abrupt floodwaters throughout the system that ultimately cut off tour participants from a feasible escape route, trapping them below, according to media outlets.

Among those still missing is the tour's leader, identified by media outlets as Konstantin Filipov, 31. The Daily Beast and The Daily Mail reported that Filipov clocked the heavy rains during the tour and texted a colleague in hopes of finding a new exit minutes before disaster struck.

Ria Novosti reported that more than 20 people were originally scheduled to take the sewer tour, which was focused on the Neglinnaya River, a tributary of the Moskva. But with heavy rains in the forecast, several participants dropped out, leaving just eight people on the expedition.

Many of Moscow's sewers date as far back as the 19th century, and several companies offer historic walking tours of the system throughout the city.

Russian media outlets cited emergency officials who said the tour company in charge of the missing group was an illegal organization that had been operating for three years.

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