Police in the northern province of Dien Bien said "a drug organization" was directly behind the case in which eight men kidnapped, raped and murdered Cao Thi My Duyen, 22, in early February.

They also found that Duyen’s mother, Tran Thi Hien, was in the same drug ring as the eight men.

Investigators are gathering evidence on Hien in the case, whether she denies involvement or not, Dien Bien police chief Sung A Hong said on Monday.

This drug gang has operated for some time and many of its members have been arrested, he said.

"If we find more proof that ties suspects to drug-related crimes, we will expand the investigation. For now, the focus will be on the rape and murder only," he said.

The eight men involved in the rape murder – Vi Van Toan, 37, Bui Van Cong, 44, Vuong Van Hung, 35, Pham Van Nhiem, 47, Luong Van La, 26, Luong Van Hung, 39, Pham Van Dung, 47, and Cam Van Chuong, 45 – are also being investigated for robbery, storing narcotics and illegally detaining people. The eight men are all drug users.

During the investigation, these suspects had changed their testimonies several times, police say.

From L: Vi Van Toan, Bui Van Cong and Vuong Van Hung are held at a police station
in Dien Bien Province for gang rape and murder. Photos courtesy of police

Investigators found Toan had told the other seven men that Hien owed him a lot of money, so he wanted them to help him kidnap her daughter and blackmail her.

In return, Toan would give them money and drugs.

Hung came to check out chickens sold by Hien and her daughter Duyen at the Muong Thanh Market in Dien Bien Phu Town on February 4.

He asked for Duyen’s number so that he could place orders later. The same night, he called Duyen and asked her to deliver 13 chickens to a specific address in town.

As Duyen arrived with the chickens, driving a motorbike on her own, he and others in the group kidnapped her and strangled her unconscious. They also took the chickens and the bike. Then they took her to Cong’s house and from the night of February 4 to the first hours of February 7, the group raped her one after another and killed her on the morning of February 7.

They dumped her body in an abandoned house five kilometers (three miles) away.

Police found that Cong's wife, Bui Kim Thu, 44, was aware of what her husband and other men did to Duyen but did not inform anyone. She pretended to be the first to discover Duyen’s body by accident, police said, adding that she had tried to mislead the investigators. Thu is being investigated for not reporting the crime to the police.

Hien, the victim's mother, had told investigators earlier that she knew none of the men suspected to have brutally raped and killed her daughter. Police have detained her to serve their investigation.