DAEGU, South Korea — Kim Ho-keun, a 68-year-old grandfather, was about to get off the crowded subway when an explosion knocked him to the floor. He awoke in darkness minutes later, gasping for breath and desperate to escape a fiery attack that killed 120 people on yesterday.

Struggling to call for help, Kim feared the worst until he heard a rescue worker’s voice.

“I couldn’t see him, but I saw his flashlight, so I grabbed his hand,” Kim said from his hospital bed, tubes supplying him with oxygen. “It was then that I thought to myself: I’m going to live.” Kim was one of the fortunate in Daegu, South Korea’s third-largest city.

The fire started about 10 a.m. when a man lit a container of flammable liquid and tossed it. The blaze incinerated two six-car subway trains, killed 120 people and injured 138, many of them seriously.

Rescue workers gave up the search for survivors by the afternoon.

A suspect who police say has a history of mental illness was under interrogation. Police said they did not know what motivated the attack or what substance the attacker used to start the blaze.

The fire began in one train at a station, igniting seats and spreading to another train as it pulled in, officials said. More people died in the second train because many of the doors failed to open, trapping passengers.

YTN TV news channel reported the second train arrived four minutes after the fire started. It was not clear why the second train was not warned of the fire or diverted from the station.

Lim Dae-yoon, chief of Daegu city’s east district municipal government, estimated the number of people killed at 120. “We believe the death toll will not rise drastically from that,” Lim said. Many bodies were burned beyond recognition. Officials said they would have to wait for DNA tests to confirm the number of dead, which could take weeks.

Other people died of asphyxiation on the train platform. One man said his missing daughter called by mobile phone to say there was a fire and the subway door wasn’t opening.

Firefighters gave horrifying accounts of the scene underground. Many bodies were found on the subway stairs, where people apparently suffocated as they tried to escape. On the platform and in the trains were the ashen bones of those trapped in the flames.

Chung Sook-jae, 54, rushed to the scene after her daughter, 26-year-old Min Shim-eun, telephoned her husband to say she was choking. Then the line went dead.

“She never caused any problems. She was a good kid. Why does this have to happen to her?” Chung cried. “If she’s not out by now, she’s probably dead. What am I going to do if her body is all burned out of recognition?”

Officials said that the fire was put out by 1 p.m., about three hours after it started, but toxic gas from the fire delayed rescue efforts.