An 81-year-old patient with senile dementia was taken into custody Wednesday on suspicion of starting a fire that killed of 21 people at a hospital for the elderly in Jangseong, South Jeolla Province.

The suspect, identified by his surname Kim, was caught on security camera as he entered the room where the fire began. Kim reportedly claimed to have no knowledge of the fire, and the authorities were attempting to determine if his actions were affected by his illness.

The fire at the hospital broke out in the early hours of Wednesday, killing 20 patients and one nurse on night duty. 

The fire was first reported at about 12:27 a.m. Fire fighters arrived at the scene within four minutes of the fire being detected.

The blaze was put out by 12:55 a.m., but most of those who died had been unable to evacuate due to old age and medical conditions.

Most of the victims are said to have been in their 70s and 80s, and suffering from senile dementia or the effects of stroke.

According to reports, there were 35 people on the second floor of the building when the fire broke out, seven of whom managed to escape.

The authorities are also investigating whether the hospital exercised the practice of placing patients in restraints, based on witness accounts.

Separately, a 71-year-old man identified by the surname Cho was taken into custody on suspicion of carrying out arson on a subway train in Seoul later in the day.

No serious injuries have been reported. The suspect was taken into custody as he was receiving treatment for burns.

Cho allegedly started the fire to commit suicide in protest of a settlement he recently received. According to the police, Cho received a settlement for damages dealt to a business he ran 15 years ago.

There were about 370 passengers on board the train, 50 of whom were in the train car where Cho set the fire. The fire was extinguished by subway officials within six minutes. Other than the arson suspect, a woman in her 60s was injured while evacuating the train, officials said.

By Choi He-suk (cheesuk@heraldcorp.com)