A fire at a Moscow hospital for drug addicts has killed 45 women and at least eight more suffered carbon monoxide poisoning or other injuries.
Distraught relatives of patients gathered outside the clinic
Officials say they are "90% certain" arson caused the overnight fire at Hospital 17 in the city's south-west.
All appear to have died of smoke asphyxiation before rescuers arrived.
They were trapped between fire at one end of their second-floor corridor and a locked metal grille barring stairs at the other end, an official said.
The Moscow fire was followed on Saturday by a second lethal fire, in a hospital in the Siberian town of Taiga.
Eight people died at the hospital, also for drug addicts, in the province of Kemerovo.
Windows in the five-storey Moscow hospital building were barred and fire safety concerns had been raised before.
The fire, which broke out about 0140 (2240 GMT) in a kitchen, was relatively small but there was very heavy smoke, possibly from burning plastic wall coverings, Russian media report.
"Everyone who died in this fire, was dead before the first fire engines arrived," said Deputy Emergencies Minister Alexander Chupriyan.
He added that crews had responded to an emergency call in just four minutes, evacuating the hospital which had a total of 177 patients and 15 staff at the time of the fire.
In daylight, relatives and friends of the dead women, who included patients infected with Aids/HIV, could be seen weeping outside the building on Bolotnikovskaya Street.
As an investigation was launched, a spokesman for the Moscow fire brigade accused hospital staff of being slow to raise the alarm.
Fire safety officers had called for the hospital's closure
"[They] worked very badly - they did not take steps to evacuate people in the early stages of the fire," Yevgeny Bobylyov was quoted as saying by the Associated Press news agency.
However there were suggestions that staff had been overcome so rapidly by smoke that they had been incapable of taking action.
Televised footage shows scorched, peeling corridor walls inside the building and beds and personal effects covered in black ash in a room which looked otherwise undamaged.
"The fire was relatively small," one rescue official told Itar-Tass news agency.
"But there was very thick smoke, and people got poisoned by smoke in their sleep."
Mr Chupriyan said there was evidence of a desperate struggle by patients to escape:
"Judging by the placement of the bodies, they really tried to get out."
Andrei Kotov, a male patient on the 4th floor, said no fire alarm had been sounded and staff had come in to wake people in his ward.
Speaking to Interfax news agency, he said he could hear cries for help from the second floor "but evidently nobody did go to the aid of those women".
Fire safety officers visited the hospital in February and March and called for its temporary closure after their second visit, said Russia's chief fire inspector, Yuri Nenashev.
"Unfortunately this decision was not adopted," he added.
Irina Andrianova, a spokeswoman for the emergencies ministry, said the design of the building was "very particular" because it was used to treat drug addicts.
Mr Nenashev said the fire had begun on a wooden shelf, which led him to suspect arson or "extremely careless handling of flammable materials".
Russia records about 18,000 fire deaths a year, AP reports - 10 times more than in the US.
Last December seven people died in a fire at an institute for the mentally ill near Moscow.
And in 2003, 36 students died and some 170 were hurt at a hostel for foreign students.