The operators of some 850 coal mines who are accused of tampering with dust samples used to gauge miners' risk of black lung disease are being fined a record $5 million, Labor Secretary Lynn Martin said Thursday.

The announcement caps a 20-month investigation of the coal mining industry in which federal agents, according to the Labor Department, uncovered about 4,700 instances of dust sample tampering.Martin said she was especially troubled by the "broad nature" of the abuse. The tampering was found at nearly half the nation's coal mines.