Illegal deforestation in and around the reserves threatens the butterflies, which rely on the forest cover to protect them from the cold, high-altitude winds. Huge numbers of Monarchs died during a cold snap in 2002.
The logs and lumber seizure was the equivalent of about 600 heavy truckloads of logs, the attorney general's office said, calling the Wednesday raid "the largest seizure of illegally logged wood in the country's history."
"These actions by the federal government are aimed at protecting the natural balance of the Monarch reserve, where every year millions of butterflies travel over 4,000 kilometers to winter," the attorney general's office said in a news statement.
The seizure was also an indication of the failure of highly publicized pledges by previous administrations to end illegal logging in the area.
More than 6,000 tons of wood were cut from within the Monarch butterfly reserve, the attorney general's office said.
About 700 police and environmental officials raided 19 clandestine saw mills in the western state of Michoacan on Wednesday.
Police detained 56 people, prosecutors said, including sawmill employees, lumberjacks and truck drivers, and seized scores of trucks and other vehicles.
Prosecutors have 48 hours to lodge formal charges or release the suspects. If convicted of illegal logging, they could face prison sentences of six months to nine years, and fines of up to 150,000 pesos (US$13,650; EUR 9,378).
"That's the important thing, that people are being charged," said Augusto Cabrera, a spokesman for the attorney general for environmental protection. "Before, they would seize wood and dismantle sawmills, but there weren't many charges."
In all, authorities seized about 6,000 cubic meters (7,850 cubic yards) of logs and boards, equivalent to about 6,000 metric tons (6,600 U.S. tons) of wood, or about 1,750 adult trees.
The government has closed 59 illegal sawmills and charged 193 people with related crimes in the last year, not including Wednesday's raid, Cabrera said.
A study conducted in 2000 showed that 44 percent of the fir forests that shelter the migrating butterflies during their annual stopover had been damaged or destroyed over the preceding 29 years.
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