Brasilia, Jun 20 (IANS/EFE): Brazilian police began another phase in the investigation of massive corruption at state oil company Petrobras, arresting the heads of two of the country's most important companies.

The presidents of construction giants Odebrecht and Andrade Gutierrez, Marcelo Odebrecht and Otavio Azevedo, respectively, were among the executives detained on Friday in an operation spanning four states, authorities said.

Andrade Gutierrez and Odebrecht are active in dozens of countries around the world, building everything from railways to hydroelectric dams.

A total of 27 firms are under investigation for their alleged participation in a corruption scheme that cost Petrobras an estimated $2 billion over the course of a decade.

Firms paid bribes to secure inflated contracts with Petrobras, splitting most of the extra money with corrupt executives of the state-controlled company while setting aside some of the loot to pay off politicians who provided cover for the graft.

Most of the 50 or so politicians caught up in the case are from parties supporting the government of President Dilma Rousseff.

Those potential defendants include leaders of both houses of Congress.

Already in custody is Joao Vaccari Neto, the ruling Workers Party's former treasurer, who is accused of pressuring companies with Petrobras contracts to divert part of their profits to his party.