Brazilian police have arrested 18 people as part of their on-going investigation into corruption at the state-run oil company Petrobras. The arrests come amid allegations of a widening money laundering and bribery scandal sending Petrobras shares tumbling.

Igor de Paula, the Federal Police Delegate, explained: “A large proportion of the contracts that are being investigated have been obtained from prior agreements with a group bearing all the characteristics of a cartel; and moreover a cartel with a system to divert funds to pay politicians and public officials.”

The scandal has put pressure on left-wing president Dilma Rousseff, who won the elections by a narrow margin just last month. She was chairwoman of the Petrobras board from 2003 until 2010.

She has pledged a thorough investigation into the affair.

Former Petrobras executive Paulo Roberto Costa is giving evidence to police as part of a plea bargain deal.