Russian President Vladimir Putin has ordered an inquiry into whether enough was done to prevent mass deaths in the southern flood-hit Krasnodar region.
Mr Putin demanded to know how much warning people had been given about the impending disaster.
Activists blamed the ferocity of the flood on the opening of sluice gates at a local reservoir.
A separate criminal investigation is already under way into whether the 155 deaths were caused by negligence.
"The investigative committee will check the actions of all the authorities - how the notice was given... how it should have been given and who acted how," Mr Putin was quoted as saying by the Itar-Tass news agency at a meeting in Krymsk - the worst-hit town.
He was speaking after flying over the region by helicopter to see for himself the damage that followed torrential rain.
Mr Putin later declared 9 July a day of mourning for those who died in the floods, as well as 14 Russian pilgrims killed in a bus crash in Ukraine on Saturday.
It was 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.
Source: BBC News