Russian authorities say that at least 18 people have died after drinking "dangerous" unmarked alcoholic drinks.
The beverages contained a highly toxic concentration of methanol that was three to five times the legal dose.
Russia's Investigative Committee in the central Orenburg region said that it had launched a criminal investigation.
Six people have so far been arrested on suspicion of negligence and producing or selling goods that do not meet Russian safety requirements.
One of the suspects is accused of producing unadulterated alcohol, while two others are alleged to have sold the drinks.
All of the male and female victims of the alcohol were from several villages about 1,200 kilometres southeast of Moscow.
"From October 6 to October 7, nine people aged from 36 to 72 years old died with a preliminary diagnosis of the toxic effect of ethanol," the Committee said in an initial statement.
Authorities confirmed on Friday that the death toll had now risen to at least 18 people, while another 21 people were being treated.
The Committee added that a 29-year-old citizen from Orsk had been detained as part of the investigation.
The suspect is accused of "organising large-scale illegal manufacturing and distribution of dangerous alcohol-containing products," a statement read.
A 60-year-old resident of Akzharskoye village and a 47-year-old resident of Krasnochabansky village were also detained on suspicion of selling the alcohol to the victims.
Authorities also seized a number of products from a warehouse in Orsk where the drinks were made.
"The police seized 1,279 alcohol-containing products in glass and plastic bottles of 0.5 litres of various names that did not have the required labelling under the Russian law," the Committee added.
Also in Orsk there was found a manufacturing and bottling facility, from where over 3,000 empty plastic bottles and equipment for production, storage, bottling and subsequent packaging of alcohol-containing products in large quantities were seized".
Later on Friday, police also detained two relatives of the 29-year-old suspect who were allegedly involved in the mass production and distribution of the alcohol. A 50-year-old resident was also detained at his home, which contained 132 bottles of alcoholic liquid.
The Regional governor was warned citizens not to buy alcohol while the investigation is underway.
Fatal incidents linked to the consumption of adulterated alcohol or toxic substitutes are not uncommon in Russia.
In 2016, more than 60 people died in Irkutsk, Siberia, after ingesting counterfeit hawthorn oil bath oil, which contained toxic methanol.