Police in the northern province of Hoa Binh have arrested three people suspected of being responsible for the deaths of eight patients who were undergoing kidney dialysis treatment at a local hospital.
Investigators have been building a criminal case for one of the most serious medical incidents in years.
Authorities have blamed the deaths on the water used in the dialysis treatment and on improper maintenance and operation of medical equipment.
Bui Manh Quoc, 31, director of Tram Anh Water Treatment Company, is being investigated for manslaughter.
Tran Van Son, a staff member responsible for medical equipment at Hoa Binh Province General Hospital, and Hoang Cong Luong, a nephrology doctor, have been arrested for medical malpractice.
On May 29, 18 patients were undergoing kidney dialysis treatment at the hospital, 80 kilometers (50 miles) southwest of Hanoi.
After 45 minutes some reported nausea, abdominal pain and shortness of breath.
The hospital sent for help from Hanoi but it was too late for eight of the patients, who appeared to have died from anaphylactic shock. The remaining 10 patients were transferred to Hanoi's Bach Mai Hospital and have recovered from the incident.
Truong Quy Duong, the hospital's director and Do Thi Diep, a nurse, have also been suspended for the investigation.
The Ministry of Health has promised "quick answers" for the case.