Police investigating last week's kidnapping and murder of a woman have booked one more suspect as they are broadening the inquiry to uncover possible masterminds and additional accomplices behind the crime, officials said Wednesday.

The Suseo Police Station in southern Seoul has detained three suspects in the death of the 48-year-old woman and requested an arrest warrant for another accomplice.

An official said they have booked a fifth suspect in the case.

"So far, five suspects have been booked and five people have been banned from leaving the country," the official said.

"We are conducting a broad investigation to identify any additional accomplices and possible masterminds behind the crime," he added.

While police did not specify who the new suspect is, informed sources suggest it may be a woman surnamed Hwang or her husband Yoo, who allegedly gave tens of millions of won to one of the three arrested men, surnamed Lee, before the incident.

The police are looking into whether the payment is linked to the crime and have barred the couple from leaving the country.

According to sources, Hwang and Lee have had a close relationship since a 2021 criminal case related to their cryptocurrency investment, which also involved the murdered victim.

According to legal sources, Lee, the murder victim and Hwang were co-investors in a cryptocurrency in 2020-21.

After the digital currency fell sharply in February 2021, Lee, the victim and other investors believed Hwang manipulated the token's price. They extorted 190 million won (US$144,460) worth of cryptocurrency coins from her.

Lee and two other suspects were apprehended Friday for kidnapping the 48-year-old woman near an apartment building in Gangnam Ward on Wednesday night, killing her several hours later and burying her body on a hill in Daejeon, about 140 kilometers south of the capital. They were formally arrested Monday.

They allegedly committed the crime to steal her cryptocurrency assets.

The police on Tuesday requested an arrest warrant for a man suspected of helping prepare the crime.

Lee denied his involvement in the kidnapping and murder, but the two other accomplices have admitted to the crime.

The two claimed Lee proposed the crime to steal the victim's money and that they planned it for two to three months.

On Tuesday, police raided a plastic surgery clinic in Gangnam and interviewed Lee's wife, who works there, to determine the source of the syringes and liquid presumed to be anesthetic found in the car used in the kidnapping.

Source: Yonhap News Agency