BANGKOK - Thai authorities on Monday confirmed all the 13 Chinese crewmen on two hijacked ships were killed and identified the killers as a drug ring on the Mekong River in north Thailand.

Investigators suspected that a drug ring of Shan, or Tai Yai, ethnic group led by Nor Kham was responsible for the incident, Seramsak Seesan, chief of Chiang Rai's Chiang Saen District, where the incident occurred, told Xinhua.

Seramsak's words echo the investigation of the army. Maj Gen Prakarn Chonlayuth, commander of the Pa Muang Task Force, Thailand 's border army, said the Nor Kham drugs gang killed all the crew members of any vessel which refused to pay the gang protection money. He believed that that is what happened in this case.

Nor Kham, 40, wanted on Thai and Burmese to arrest warrants for drugs trafficking, had expanded his illegal activities to collect protection money from Chinese-flagged cargo ships a few years ago, Prakarn said.

The gang hijacked ships plying the river and demanded protection money from them. If they refused to pay, they kill the crew and take over the ships to deliver drugs from Burma to Thailand, he added.

About 400 armed men are thought to belong to the Nor Kham drugs gang, said Permpong Chavalit, deputy secretary-general of the Office of the Narcotics Control Board.

Searchers found the body of the last Chinese crewman in nearby Chiang Khong District early on Monday.

Last Wednesday, Thai border troops seized drugs on board two Chinese-flagged ships coded Yi Xing 8 Hao and Hua Ping, after a gunfight of more than 30 minutes with drug traffickers, in Chiang Saen District, bordering Myanmar. One drug trafficker was reported to have been killed.

The police found bodies of the Chinese sailors late last week. Three bodies, with hands tied and handcuffed behind their backs, were found on Friday. Another nine bodies were found on Saturday.

The bodies were identified as the crew of Yu Xing 8 Hao and Hua Ping. The police suspected that drug traffickers had planned to use the hijacked ships to smuggle drugs into Thailand and the sailors were killed before the gunfire on Wednesday.

Autopsies are being conducted in Chiang Saen Hospital. Relatives of the sailors will arrive on a later date to cooperate in investigation and claim the bodies, Seramsak said, adding that this was the first time a commercial ship was hijacked by drug traffickers.

A committee of police, military and local authorities are further investigating the case, Seramsak said.

The case attracted extensive attention from China. The country's Foreign Ministry on Sunday urged the related authorities to find out the truth and arrest the killers as soon as possible and ensure safety along the Mekong River.