Eleven people were drowned and another 10 are missing after a boat believed to be carrying migrants sank off the Mexican state of Oaxaca.
Those on board were thought to be trying to get to the United States from Central America.
Two survivors, both from El Salvador, said that the boat had sailed from Guatemala on Tuesday.
Initial reports said that 24 bodies had been found. A tropical storm is now hampering the search for survivors.
"There are reports of more bodies, but the weather hasn't let us go out to sea," said Oaxaca state public safety secretary Sergio Segreste.
One survivor, Noemi Martinez, 29, told local TV that after the boat was wrecked she survived by clinging to a barrel.
At this stage, bad weather is being blamed for the sinking, but boats carrying illegal immigrants through Mexican waters are often overloaded or unseaworthy.
Civil protection authorities in Oaxaca received calls that a boat carrying illegal immigrants from Central America had capsized off the Pacific coast.
"We got a report that a vessel carrying undocumented migrants had capsized or gone down," Mr Segreste told the Associated Press news agency.
"The assumption is that the cause of the accident was the rough weather."
Correspondents say there are numerous ways in which people-smugglers take immigrants to the United States via Mexico and traffickers are constantly seeking new routes which avoid checkpoints and customs officers.
Often people are carried first to Mexico by boat, from where they must make a second hazardous journey across the border hidden in trucks or lorries.
Other journeys involve hundreds of kilometres by sea to take immigrants directly to the US coastline.
Many Cubans who leave the island travel to Mexico first rather than trying to cross the Florida straits. Illegal immigrants often pay as much as $5,000 to be taken north.
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