(Adds prosecutor quote, context)
By Gram Slattery and Marta Nogueira
RIO DE JANEIRO, Nov 21 (Reuters) - Swiss investigators have executed search warrants at addresses linked to Vitol and Trafigura, their counterparts in Brazil said on Thursday, as a sprawling probe into the global commodity trading industry intensifies.
In a statement, Brazilian federal prosecutors said Swiss investigators searched the Geneva addresses on Wednesday as part of a probe made public late last year. As part of that probe, authorities alleged employees of major commodity trading firms paid employees of Petroleo Brasileiro SA at least $31 million in bribes from 2011 to 2014.
In return, the trading firms would buy Petrobras fuel for artificially low prices or sell at artificially high prices, fleecing the state-run company in the process.
One employee of Petrobras, as the firm is commonly known, has pled guilty in the United States to conspiracy to commit money laundering and is cooperating with the U.S. Federal Bureau of Investigation in a parallel investigation of the matter.
In September, Reuters reported that a cooperating witness in Brazil told prosecutors that high-ranking executives at Vitol, Trafigura and Petrobras were aware of the scheme, and in some cases helped facilitate bribe payments.
In a statement, Brazilian federal prosecutors said the Swiss search warrants were key, as investigations indicated there was important incriminating documentation there.
"The collection of this evidence will help hold members of the upper echelon of Trafigura and Vitol responsible, as well as middlemen and public workers split up among different areas of (Petrobras') trading division," federal prosecutor Athayde Ribeiro Costa said in a statement.
Petrobras did not immediately respond to a request for comment.
Vitol said it has a "zero tolerance policy in respect of bribery and corruption" and cooperates "fully with the relevant authorities in all the jurisdictions in which we operate."
Trafigura said it had responded to the request by Swiss authorities for information "and provided available responsive documentation."
The trading house added that it "is continuing to keep these matters under review and is taking the allegations previously made seriously."
Petrobras previously said it has carried out internal investigations into the matter and has removed those suspected of wrongdoing. The company has repeatedly said it is committed to rooting out graft. (Additional reporting by Carolina Mandl; Editing by Peter Graff)