MIAMI BEACH, Florida (AP) — The Coast Guard said it suspended its rescue operations at sunset Thursday after announcing earlier that afternoon that it had found four additional bodies in its search for dozens of migrants lost at sea off Florida.

Homeland Security Investigations officials have said they were actively investigating the case as a human smuggling operation.

Authorities have now found a total of five bodies, leaving 34 missing five days after the vessel capsized on the way to Florida from Bimini, a chain of islands in the Bahamas about 55 miles (88 kilometres) east of Miami.

Coast Guard Captain Jo-Ann F Burdian said earlier the decision to suspend the search was not an easy one.

“We have saturated the area over and over again,” she told a news conference. “We’ve had good visibility. … We’ve overflown the vessel a number of times. … It does mean we don’t think it’s likely that anyone else has survived.”

The Miami office of Homeland Security Investigations has launched an inquiry, saying the migrants’ journey was most certainly part of a human smuggling operation. Under federal law, a smuggler convicted of causing a death is eligible for execution.

“The goal of this investigation is to identify, arrest and prosecute any criminal or criminal organisation that organised, facilitated or profited from this doomed venture,” said HSI Miami Special Agent in Charge Anthony Salisbury.

Salisbury declined to give any information on the nationalities of the boat passengers but said investigators consider the lone survivor “a victim right now,” not a suspect. Salisbury appealed to the public for tips to help identify who organised the boat crossing.

“Please help us bring criminals who prey on and victimise the vulnerable migrant community to justice,” he said. “We don’t want anybody doing this again. … This is dangerous stuff.”

The lone survivor was found hanging onto the 25-foot (7-metre) vessel about 40 miles (64 kilometres) off Fort Pierce, Florida. He told a good Samaritan and authorities that the boat capsized late Saturday after he and 39 others had set out for Florida from Bimini.

Authorities said the boat was found about 100 miles (160 kilometres) north of where it capsized, apparently pushed by the Gulf Stream, a warm, swift current that wraps around the Florida peninsula and flows along the Atlantic Coast of the United States. No one was wearing a life jacket, the rescued man told authorities.

The Gulf Stream can be treacherous even on a calm, sunny day. Throw in an overloaded boat, inexperienced mariners, stormy weather and the dark of night, and they can become deadly.

A small craft advisory had been issued on Saturday and Sunday as a severe cold front with winds up to 23 mph (37 kph) blew through the dangerous passage, creating swells up to nine feet (three metres).