MEXICO CITY: Mexican authorities on Friday searched the offices of Buenavista copper mine, which belongs to miner Grupo Mexico, as part of an investigation into a toxic spill that could cost the firm billions of pesos, the prosecutor's office said.

Mexican officials are preparing to bring a lawsuit against Grupo Mexico, which released 40,000 cubic meters of toxic mining acid into the Bacanuchi River in the Northern state of Sonora earlier this month, Mexico's environmental prosecutor said.

A spokesman for Mexico's attorney general's office declined to specify what authorities were searching for and Grupo Mexico declined immediate comment. Metals miner and railroad operator Grupo Mexico is in the midst of a $3.4 billion expansion project at the mine, which has the largest proven copper reserves in the world.

The expansion aims to boost production capacity to 1.3 million tonnes by 2017.

Mexico's Congress earlier this month urged the government to cancel Grupo Mexico's concession to operate the mine, formerly known as Cananea.