A rescued migrant holds a Gabinetto di Polizia Scientifica (Forensic Police Regional Cabinet) number after disembarking from an Italian Coast Guard ship in the harbor of Palermo, Sicily on April 14. (Alessandro Fucarini/AP)

This post has been updated.

The 500 or so migrants — most of them young men — set off from the Libyan coast this week, encouraged by the spring-like weather and the prospect of escaping turmoil in the countries they were leaving behind.

But within 24 hours, the journey became a disaster. The ship carrying them capsized, casting everyone on board into the Mediterranean’s still-chilly waters. As of Tuesday evening, only 144 people had been rescued from the shipwreck. About 400 more are missing and presumed dead.

The incident came on the heels of a weekend in which as many as 8,500 people attempting to make it to Europe were pulled from the Mediterranean. It’s the beginning of “migration season” — the stretch of warmer months during which people fleeing conflict in the Middle East and North Africa are most likely to be smuggled across the sea to Europe. And this could be the deadliest season yet.

Already, humanitarian agencies had raised alarms about the rising number of people who have died during the dangerous ocean crossing. According the United Nations High Commissioner for Refugees, this year’s toll of dead and missing now exceeds 500 people — a 30-fold increase from the same period in 2014. That number doesn’t include the 400 missing from Tuesday’s shipwreck.

And as the summer months approach, the death toll will continue to rise, Save the Children spokesman Michele Prosperi told the BBC Tuesday.

“When the weather improves … we may have many more arrivals,” he said.

[Two German businessmen are on a daring mission to save migrants from drowning in the Mediterranean]

[How smugglers bring desperate migrants across the Mediterranean]

Conditions on smuggling ships — rickety wooden or rubber crafts of questionable seaworthiness — are cramped and chaotic. The boats are often inadequately supplied and captained by a volunteer equipped only with a compass, or GPS if he’s lucky. Smugglers have been known to drive their boats out to sea, then jump ship, assuming that their passengers will be rescued by someone else. It’s not rare for boats to crash or capsize. Even when the ships make it to the European coast intact they can still be deadly, as was the case for one migrant found dead on a boat intercepted by the Italian Coast Guard this weekend. He appeared to have been suffocated by fumes from the engine, according to the International Organization for Migration.

Last year more than 3,400 people died during the journey, Bloomberg reported, making the Mediterranean the world’s most lethal border crossing.

But refugees continue to flee across the water in record numbers. Escalating conflicts across North Africa and the Middle East have driven tens of thousands to risk the perilous sea journey. They leave from Libya, where political chaos and a few-hundred-mile journey to the toe of Italy’s “boot” have created an ideal base for smugglers. But for most, the ocean crossing is just the final leg of their trek — the majority of those crossing the Mediterranean come from Eritrea, Somalia and Syria.

“Was there really an alternative to this dangerous sea journey?” Aali, a 21-year-old Libyan who was among those rescued this weekend, told UNHCR. He told the agency that his brother had been killed and his food shop torched by militants in the chaos.

“The war changed everything,” Aali said.

[Migrants dying in Mediterranean by the thousands as they flee turmoil in Libya, Syria]

[To some, the E.U.’s deadly border is the new Berlin Wall]

European governments and humanitarian agencies have not come to grips with the crisis. Italy’s “Mare Nostrum” mission, a costly project that sent ships far into international waters to intercept smuggling boats, ended last fall when it was deemed too expensive. It has been replaced by a more limited European Union program called “Triton,” which will remain within 30 miles of the Italian coast and focus on “border management” rather than search-and-rescue missions. Meanwhile, the Italian Coast guard has responded to most of the recent shipwrecks.

Many European countries oppose programs like “Mare Nostrum,” arguing that the hope of being rescued encourages more migrants to attempt the deadly crossing. British Foreign Office Minister Joyce Anelay spelled out her country’s position last fall in a question and answer session in the House of Lords: “We do not support planned search-and-rescue operations in the Mediterranean. … The Government believes the most effective way to prevent refugees and migrants attempting this dangerous crossing is to focus our attention on countries of origin and transit, as well as taking steps to fight the people-smugglers who willfully put lives at risk by packing migrants into unseaworthy boats.”

But aid agencies and refugee advocates say that, by dismissing a robust search-and-rescue program, Europe is effectively allowing Mediterranean migrants to die. UNHCR called for stepped-up rescue efforts in a statement Tuesday, noting that “many more people will die” if more resources aren’t devoted to the issue. Some private citizens are taking matters into their own hands — like the pair of German businessmen who bought a yacht to patrol the Mediterranean and rescue refugees on their own or the family that set up a “Migrant Offshore Aid Station” (MOAS) last fall.

Doctors Without Borders announced Monday that it will partner with the MOAS to establish its own search-and-rescue operation in the absence of a more expansive E.U. effort. The program has only one ship — the 43-yard “M.Y. Phoenix” equipped with inflatable rescue boats and surveillance drones — but it’s better than nothing, the aid group said.

“It’s easy to sit on the shore and point at each other,” Hernan del Valle, head of humanitarian affairs at Doctors Without Borders, told the Associated Press on Monday. “But we can’t be letting people drown.”