Two people searching for treasure beneath an old Mexican hacienda died in a gas-filled tunnel, and two neighbors and two emergency workers who tried to rescue them also perished, authorities said Tuesday.

"It appears they were looking for some treasure, from the era of the hacienda," said Jose Luis Cruz, a police officer in the central Mexican town of San Jose del Rincon where the deaths occurred. "It's a belief around here. It is said that people have found treasure around there before."

Sergio Ramirez, director of emergency services for the State of Mexico, said a man, 65, and his 60-year-old wife were the first victims.

They were looking for loot in the 15-yard-deep tunnel, which burrowed about 50 yards horizontally underground toward the abandoned hacienda residence.

Two local construction workers entered the primitive, earthen-walled tunnel to rescue the couple, apparently unaware of a dangerous buildup of exhaust fumes from a gas-powered pump the couple had installed in the tunnel to remove water.

The two would-be rescuers also died, as did a firefighter and a paramedic from nearby towns who later entered the tunnel without oxygen tanks.

It took specially equipped rescue personnel about 20 hours to remove the six bodies.

The couple — Juan Sandoval and Maria Elena Romero — apparently had become obsessed with finding whatever it was they thought lay beneath the long-abandoned hacienda. Police said it was unclear whether the legendary loot involved money or precious metals.

"It looks like it took them several years to build this," Ramirez said. "The walls are bare earth. ... There were absolutely no safety measures."

The tunnel is about 4 yards wide at its mouth, but narrowed to spaces that rescuers had to crawl through on their hands and knees to retrieve the bodies.

San Jose police commander Marco Antonio Posadas said the elaborate project included an antique-style metal mining cart that the couple were using to remove material.

The Hacienda de San Onofre, an old farm estate located in San Jose, was owned in the 19th century by a mining magnate. While wealthy in the pre-revolution era, the area is now impoverished and depends largely on small farms.

Many old estates were abandoned during the 1910-1917 Mexican Revolution, when their rich owners fled. Legends persist about the caches of valuables left behind or looted by rebels during the revolution.