Russian authorities investigating the devastating shopping centre fire in Siberia, which left 64 people dead, have said there were "serious violations" at the centre.
The Investigative Committee said the fire exits at the centre in Kemerovo had been blocked and a security guard had switched off the alarm system.
It said four people had been detained over the fire, including the owners and occupiers of outlets inside the centre.
The committee, which handles major crimes, said it was trying to bring in the centre's owner for questioning.
The fire, which was one of the deadliest in Russia since the break-up of the Soviet Union, swept through the upper floors of the "Winter Cherry" shopping centre yesterday afternoon.
The blaze gutted large parts of the building, which also contained a cinema complex and children's play area.
Emergency services said they had extinguished the blaze, but later said it had reignited.
Rescuers struggled to reach the building's upper floors because the roof had collapsed.
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Television footage showed thick black smoke rising from the building.
It was unclear if any people were still unaccounted for, but 11 people were being treated in hospital, including an 11-year-old boy who was in a serious condition.
Earlier, people had posted appeals on social media seeking news of their relatives or friends.
Authorities set up a centre in a school near the shopping centre to deal with inquiries.
Anna Kuznetsova, Russia's children's rights commissioner, said the fire had been caused by incompetence and warned there were many similar shopping centres.
The shopping centre, a former cake factory, had few windows or doors.
Witnesses were quoted by Russian media as saying that the fire alarm had failed to go off, and that many people had found themselves trapped because exit doors were locked.
Video footage from inside the centre after the fire broke out showed a group of people in a smoke-filled staircase trying to smash a fire exit door, which was jammed.
Russia's Channel One TV station reported that some people had jumped from upper windows to escape the flames.
The Interfax news agency cited an unnamed local official source as saying the main theory being looked at was that the fire had been caused by an electrical short circuit.
However, it quoted Vladimir Chernov, the region's deputy governor, as saying that the blaze had started when a child had set fire to the foam on a trampoline in a play area using a lighter.
President Vladimir Putin "expressed his deep condolences to the relatives and loved ones of those who died," the Kremlin said in a statement.
It was the deadliest blaze in Russia in recent years.
A shopping centre fire in March 2015 killed 11 people in Kazan, the capital of Tatarstan some 800km east of Moscow.
In April 2013, a fire ravaged a psychiatric hospital in the Moscow region, killing 38 people, most of them patients who were engulfed by flames as they slept behind barred windows.
Just months later, in September 2013, 37 people were killed when a fire swept through a psychiatric hospital in the village of Luka in northwest Russia.
In 2009, 156 were killed in a nightclub fire in the city of Perm, 1,200km east of Moscow in one of the deadliest accidents in Russia's modern history.