The side of a sodden mountain collapsed on a bus carrying up to 60 passengers along a remote Mexican road on Wednesday and hours later rescuers could only pull a woman's corpse from the debris.
Local rescuers said those on board were probably killed but the government held out hope for survivors.
After days of heavy rain, part of an almost vertical hill tumbled on the bus as it wound through an indigenous area of the southern state of Puebla, burying it entirely under a sea of mud, fallen trees and jagged gray boulders.
A second smaller mudslide hampered early efforts by bystanders to dig for survivors at the disaster site in the municipality of Eloxochitlan.
As much as 7 metres of mud and rock lay on top of the vehicle, senior state rescue official German Garcia told Reuters.
Soldiers heaved at the debris by night with digging vehicles but were only able to reach the body of a woman thought to be in her mid-twenties, according to state officials. No survivors were found.
A few hundred villagers stared at the rubble while a handful prayed around candles in a nearby house.
"We're so sad, we're destroyed. Just think how many souls we have lost," said 30-year old corn farmer Benito Cortes.
The federal government said the bus was carrying 60 people, although Garcia estimated closer to 45 victims were buried.
President Felipe Calderon sent orders for the interior and defense ministries to join in the effort to dig out the bus, which had left the town of Porfirio Diaz bound for Tehuacan.
Mud and debris from the slide covered 200 sq. yards around the bus. Rescuers feared the passengers were suffocated or crushed.
"It's very unstable zone, you have to move very carefully," state official Garcia said.
Puebla state is home to one of Mexico's largest cities, but also includes vast cacti-studded deserts, rolling plains and remote indigenous mountain hamlets where little Spanish is spoken and locals scrape a living farming coffee and corn.
Relentless rain in recent days with the start of the annual rainy season has caused flooding in many parts of Mexico.