ISLAMABAD -- Pakistan on Monday widened its search for human traffickers after a tragedy off the Greek coast last week that left more than 500 migrants feared drowned, including many Pakistanis, officials said.

With no definitive casualty numbers announced, families feared for the fate of their loved ones and the nation observed a day of mourning Monday, declared by Prime Minister Shahbaz Sharif's government.

The Pakistani flag was flying at half-staff, and lawmakers in the National Assembly, the lower house of the parliament, expressed their condolences.

The fishing trawler packed with migrants overturned and went down early Wednesday off southwestern Greece in one of the deadliest-ever incidents in the central Mediterranean Sea. The vessel was carrying as many as 750 people, including scores of Pakistanis, when it sank in international waters. A search-and-rescue operation has since been underway.

Pakistan's Embassy in Athens has so far identified 12 Pakistani nationals rescued by the coast guard, but no information was available about those who went missing after the vessel sank.

Pakistani police said they were interrogating three arrested traffickers in connection with the sinking. Desperate for a better life, many Pakistanis pay up to $8,000 to traffickers to smuggle them to Europe through Iran, Libya and Turkey.

Meanwhile, relatives of the missing were praying for their safety. Student Sawan Raza, 20, said his brother, Ali Reza, 28, had tried to make it to Europe with the help from human smugglers, to find a better job.

鈥淲e are waiting for a miracle and miracles do happen,鈥 Sawan told The Associated Press by phone from the city of Gujrat in Punjab province, from where an estimated 50 people are missing.

Some of Pakistan's tragedies in migrant crossings have been widely publicized. Pakistani national soccer team player Shahiza Raza died in March in a shipwreck off Italy's southern coast when she embarked on the dangerous voyage to Europe find medical treatment for her disabled 3-year-old son.

Pakistan's Interior Minister Rana Sanaullah Khan said the country's Federal Investigation Agency launched a crackdown against traffickers, arresting key suspects in the eastern city of Lahore and in Karachi, the capital of southern Sindh province.

鈥淎ll the people involved in this tragedy will be brought to justice,鈥 the minister pledged in a statement, adding that Sharif's government will further toughen existing laws to include harsh punishments for human traffickers.

So far, authorities have detained nearly two dozen suspects, including two key suspected traffickers in Pakistan, and at least 12 people involved in sending young men to Libya for the onward journey to Europe.

Senior Pakistani officer Khalid Chauhan said Sunday that police picked up the suspects amid the crackdown on traffickers and were interrogating them for their alleged roles in luring, trapping and extracting huge amounts of money to send the men abroad.

Abdul Jabbar, a top official at the Federal Investigation Agency, appealed to families of those who died in the boat incident or went missing to come forward and share information about the smugglers.

He promised the families of the victims would not be arrested but 鈥渞ather they will be treated as victims鈥 themselves.

Pakistani officials also collected DNA samples from relatives and said they would send the results to Greece to help identify victims. Since the news of the incident spread, people have been offering their support to relatives of those presumed to have been on the boat. Some of the survivors have contacted their families, narrating how the boat sank.

Associated Press writer Babar Dogar in Lahore, Pakistan, contributed to this report.