ROME — Having floated for at least two days in the choppy Mediterranean Sea to reach Europe, a rickety trawler overstuffed with African migrants fleeing war and poverty was nearing a Sicilian island, not even a quarter-mile away. But it was still dark and no one had yet spotted them. So to signal their position, someone set a match to a blanket.

But rather than sending a signal, the fire brought tragedy when flames from the burning blanket ignited gasoline. Nearly 500 people are estimated to have been on board — including children — and the blaze created a panic that capsized the boat. So close to reaching land, the migrants were now tossed into the sea. Many could not swim.

The accident, which occurred before dawn on Thursday within easy eyesight of the island of Lampedusa, is one of the worst in recent memory in the Mediterranean: at least 111 people were reported dead, with up to 250 still missing. At least 150 others survived, and Italy’s Coast Guard was continuing to search for more survivors.

The grisly deaths again underscored the dangerous, desperate efforts by many migrants from Africa and the Middle East to reach Europe by sea, while also renewing criticism of European immigration policy. Immigration is a politically volatile issue in Europe, so much so that Greece recently completed a nearly eight-mile fence blocking its border with Turkey, an attempt to shut down a major land migration route.