Authorities in the U.K. are searching for two brothers wanted in connection with the 39 dead bodies that were found in the back of a tractor-trailer near London last week.
Ronan Hughes, 40, and Christopher Hughes, 34, both from Armagh in Northern Ireland, are wanted on suspicion of manslaughter and human trafficking, Essex police said in a statement.
"Finding and speaking to the Hughes brothers is crucial to our investigation," said Stuart Hooper, the detective chief superintendent who is leading the investigation.
Authorities believe the pair are in Northern Ireland but also have links to the Irish Republic.
"This is a case where 39 men and women have tragically died and support from the community is going to be vital to help bring those responsible to justice," Hooper said.
A 25-year-old man has already been charged in connection with the case and appeared in court via video link on Monday.
Maurice Robinson, who is from Northern Ireland, is charged with 39 counts of manslaughter, conspiracy to traffic people, conspiracy to assist unlawful immigration, and money laundering. Prosecutors alleged he was part of "a global ring" involving "the movement of a large number of illegal immigrants into the U.K."
Robinson, who was arrested last Wednesday and identified by police as the driver of the truck, was not asked to indicate pleas and is next scheduled to appear in court on Nov. 25.
Three other suspects, two men and a woman, were also arrested last week and questioned on suspicion of manslaughter and conspiracy to traffic people, but they have not been charged. Two were released on bail until Nov. 11 and the third has been bailed until Nov. 13, according to Essex police.
Emergency services were called to an industrial park in the town of Grays in Essex, southeast England, at around 1:40 a.m. local time on Oct. 23 when the vehicle was discovered to have people inside. Thirty-nine people -- eight women and 31 men -- were pronounced dead at the scene, which was about 20 miles east of London.
Each victim “appears to have a bag of some description, clothes, and other belongings,” according to Essex police.
“This is the largest mass fatality victim identification process in the history of Essex Police,” the department said in a statement Saturday. “So far we have over 500 exhibits, including mobile phones which have to be downloaded and the interrogation of mobile phones will be important for identifying the victims but also assisting the wider investigation.”
Essex police initially said that the victims are all believed to be Chinese nationals. But over the weekend, amid reports that some of the victims may have been Vietnamese, police noted that "as our investigations continue, the picture may change regarding identification."
“We are working hard to understand how the 39 victims of this tragic incident have died and to identify all those involved," Essex Police Detective Chief Inspector Martin Pasmore said in a statement Saturday. “We remain open minded as to nationalities of those who have died. We are asking anyone who may have information that may assist us in identification to come forward to us."
Police in Vietnam have taken hair and blood samples from family members who fear that their relatives may be among the victims.
On Saturday, Vietnamese Prime Minister Nguyen Xuan Phuc ordered local authorities to establish whether any of the 39 people found dead in the United Kingdom were in fact Vietnamese. He also ordered police to begin an investigation into alleged human trafficking activities, according to Reuters.
ABC News' Morgan Winsor contributed to this report.