Russian officials failed to give adequate warnings before flash floods that killed at least 171 people in the southern Krasnodar region, the Russian government says.
President Vladimir Putin has demanded a full report by the weekend on the disaster and how it was handled.
A day of mourning is being held for the victims. Record torrential rain has been blamed for the floods.
The town of Krymsk was devastated and the district boss has been sacked.
Seventeen people are still missing after Friday night's disaster. Russian media say 210 people are in hospital, including 48 children.
Emergencies Minister Vladimir Puchkov said there had been warnings but "not all the population was warned in time".
"Mistakes were made by local leaders and various services," he said.
Local officials say flood warnings were given using sirens, sms messages and loudspeakers. But many people were asleep at the time and did not hear them.
A criminal investigation is under way into possible negligence.
The rains dumped as much as 28cm (11 inches) of water over Friday night, forcing many residents to take refuge in trees or on house roofs. The Russian weather forecasting centre said the rainfall was five times the monthly average.
The centre's chief, Roman Vilfand, said the water poured down a steep-sided valley, rapidly swelling a river which swept away all in its path, Interfax news agency reported.
Dam 'not to blame'
Officials deny allegations - widespread among locals - that the opening of reservoir sluice gates was to blame for the ferocity of the floods.
Mr Puchkov said "there were no factors to cause any rapid rise in water levels in the reservoir", adding that he had inspected the sluice gates.
An environmental official, Nikolai Kutin, echoed his finding, saying only a breach in the dam could have produced a flood of water and that had not happened.
Earlier, an investigator said there had been "normal, planned" discharges of water from the reservoir, which could not have led to the disaster.
The day of mourning and inquiry were called by President Putin after he toured the affected area in a helicopter on Saturday.
Three government planes carrying more rescuers and relief supplies arrived in Krasnodar on Monday.
It is the first major disaster in Russia since Mr Putin, 59, returned for a third-term earlier this year, after a four-year term as prime minister.
The head of Krymsk district council, Vasily Krutko, was dismissed. Earlier it was reported that the town's mayor too had been fired, but that was later corrected.
Russia is also mourning 14 pilgrims killed in a bus crash in Ukraine on Saturday. The Russian bus was travelling in Ukraine's Chernihiv region, about 150km (90 miles) northeast of Kiev, when it veered off the road and overturned. Twenty-nine of the passengers, en route to a monastery, were injured.
TV pictures from Krasnodar region showed thousands of houses almost completely submerged, with people scrambling on to their rooftops to escape the rising waters.
Most of those who died were in and around Krymsk, a town of 57,000 people. But deaths were also reported in the Black Sea resort of Gelendzhik and in the port city of Novorossiysk, a major oil terminal.
Krasnodar governor Alexander Tkachev said more than 24,000 people had been affected by the floods. The regional authorities say more than 5,000 houses were inundated.
"It's an unprecedented tragedy. There has been nothing like it in our history," Mr Tkachev said.
He acknowledged that officials had known of the danger facing Krymsk three hours before the floodwaters hit. Addressing an angry crowd he said: "in those three hours were we supposed to visit everyone and warn them? How could we? And if we had done would you really have left your homes?"
Even Russian newspapers normally considererd pro-government were scathing in their criticism of authority, the BBC's Steve Rosenberg reports. Izvestiya wrote "the tragedy of Krymsk was a perfect demonstration of what slovenliness and hoping against hope can lead to".
More than 7,000 Russian children were attending summer camps in the area and one of the camps was evacuated, Russian media reported.