The arrested hostage-taker in Monday’s siege in Ansan, south of Seoul, stabbed a knife into the neck of his stepdaughter, one of the four hostages, and then strangled her to death before the five-hour standoff with police began, investigators said Wednesday.

The 47-year-old man, identified only by his surname Kim, was nabbed Tuesday after killing his wife’s ex-husband and the 16-year-old stepdaughter while taking the four hostages. The other two hostages were rescued unharmed.

The authorities said the stepdaughter had been stabbed to death at around 9:36 a.m. when the police received a report from Kim’s wife. The unharmed stepdaughter also stated that the suspect killed her sister in the morning as the killer got enraged at his wife for having been out of touch with him, the police said. Kim demanded that his wife come and see him, they added.

The police’s announcement came after criticism grew over the police’s rescue operation. Some claimed that the teenage victim could have been saved if the rescue mission had begun earlier. The stepdaughter victim was pronounced dead upon arrival at a hospital.

The police said they started intervening in the case from 10:15 a.m. with the police commandos standing by on the roof. The police added the commandos did not storm the scene until 2:30 p.m. because the hostage-taker was highly agitated.

Officers said they didn’t know until 12:45 p.m. that the hostages were harmed. Kim showed the body of the victim to her mother through video call at that time, they said.

The police also revealed that the biological father was killed at around 9 p.m. the previous day when the killer entered the victim’s house. The 49-year-old victim was with his girlfriend at that time. Two daughters came home later after their birth father was killed in a fight with Kim, the investigators said. The former husband was found dead in a bathroom.

The killer confessed to starting the hostage situation because he thought his estranged wife rekindled her relationship with her former husband, the police added.

Kim and the 43-year-old married in 2007 but have been separated since August last year.

The investigators said they would seek an arrest warrant for Kim on murder charges.

By Lee Hyun-jeong (rene@heraldcorp.com)