Seven people are being held by police following the deaths of 19 cockle pickers in Morecambe Bay.
A total of 19 people died
Two men gave themselves up to Lancashire Police on Monday afternoon.
Three men and two women, who were among the survivors, were arrested on Sunday. All seven are being held on suspicion of manslaughter.
The coroner has set up a commission to identify the mainly Chinese cockle pickers who died after being caught by high tides - none have been named.
Magistrates have given Lancashire police a further 36 hours to question the five people arrested on Sunday.
A group of more than 30 cocklers were trapped by rising water in the Hest Bank area of the Lancashire bay on Thursday night.
Cantonese and Mandarin translators are helping police question some of the survivors, as well as the Chinese consul general and other officials.
But the Preston and West Lancashire coroner Howard McCann said the language barrier and "absence of relatives" in the UK was making identification "very difficult".
Lancashire Police Assistant Chief Constable Julia Hodson said the process could take "weeks and possibly months".
She urged "relatives, friends, colleagues who might be able to help with identification to come forward".
Officers raided several addresses in Merseyside over the weekend.
ACC Hodson said more searches of addresses would take place as part of a "massive inquiry" to "establish the background" of the cocklers' employers.
"We still have a number of leads to follow... predominantly in the Merseyside area," she added.
"We will not spare ourselves in every effort to bring anybody we need to justice."
She added that a meeting with other emergency services, people involved in the cockling industry, immigration officials and Customs and Excise on Monday afternoon to try and ensure a similar tragedy never happens again had been "very constructive".
In an emergency statement in the House of Commons on Monday, Rural Affairs Minister Alun Michael said criminal gangs could not be allowed to continue to put workers' lives at risk.
He said: "The government wishes to see an end to the misery and exploitation unscrupulous gangmasters operating outside the law can cause.
"The government will ensure the full force of the law is brought to bear on those committing these crimes."
Mr Michael told MPs the tragedy involved the largest number of work-related deaths since the 1988 Piper Alpha disaster, when 167 workers died in a North Sea oil rig fire.
Mr Michael later told BBC News: "One of the problems in tackling the activities of gangmasters is there is a conspiracy of silence - it is nobody's interests to speak.
"And I hope one of the things that will come out of this tragedy is people will come forward, provide information to the authorities and help us to do what we are trying very hard to do - tackle the underlying causes and those who are exploiting others."
Police issued two numbers in connection with the incident:
For witnesses or those with information - 01524 63333
For worried relatives - 0870 9020999