By Caroline Stauffer

SAO PAULO (Reuters) - A Brazilian police officer investigating a kick-back scheme at state-run oil company Petrobras said the total amount of money involved could exceed $28 billion, making it the country's biggest-ever money laundering probe.

"If you consider the value being investigated, the value of the Petrobras projects, certainly the potential of the investigation is to surpass that value by a lot," Federal police agent Erika Marena said in a rare telephone interview.

Marena, speaking from the southern city of Curitiba where the probe is based, said police are investigating the possible involvement of more foreign companies and 750 infrastructure projects in addition to contracts involving Petroleo Brasileiro SA, as the firm is called formally.

Though she could not yet say exactly how much money may have been laundered in the kickback scandal, she said the case is likely bigger than the nearly $30 billion found to have been illicitly transferred abroad in a separate case between 2003 and 2007. She was also involved in that case, known as Banestado, Brazil's biggest money laundering incident to date.

The investigation has become the biggest crisis yet for President Dilma Rousseff. It has threatened to halt much-needed construction projects due to the alleged involvement of top engineering contractors and has reduced the market capitalization of Petrobras by nearly half in two months.

Marena spoke to Reuters late on Tuesday, before police arrested Petrobras' former international director Nestor Cervero. Prosecutors accused him of continued involvement in corruption and money laundering, although his lawyer said he had committed no crime.

Prosecutors last month charged Cervero with taking kickbacks from Korean company Samsung Heavy industries, which has not responded to requests for comment, and others to secure contracts with Petrobras off the coast of Africa and the Gulf of Mexico.

Marena said other international companies may have been involved as well.

"I can't give names but there are indications that other big international companies were paying bribes to obtain contracts here in Brazil," she said.

Brazilian lawyers have said foreign companies could be prosecuted here under Brazil's new anti-corruption law, which took effect in January 2014.

BEACON HILL FAME

The police and a task force of ambitious young prosecutors in Curitiba first gained international fame after their work tied Brazilian money changers to the Beacon Hill money laundering scheme in New York a decade ago.

Their investigation was a key part of the first major money laundering case after the Sept. 11 attacks on the United States, according to 2004 senate testimony from former New York District Attorney Robert Morgenthau.

They hope the current investigation will help end the widespread practice of paying bribes in Brazil and also strive to get stolen funds back, something they struggled with previously.

"We were chasing the past in Banestado," Marena said, referring to the case named for the now-defunct state bank through which much of the funds were laundered. That investigation ultimately saw some $1 billion returned.

"Now we are working with current facts, with accounts that are still open," she added.

One of the main operators in Banestado, black market money changer Alberto Youssef, led investigators to the Petrobras scheme, according to prosecutors and police.

Police discovered the link after they started investigating 10 billion reais ($3.8 billion) of suspicious financial transactions involving Youssef that was flagged by Brazil's financial intelligence unit COAF in March, Marena said.

Youssef is one of 39 people, including former Petrobras executives and leaders of Brazilian engineering firms, whom prosecutors charged last month with forming a cartel to funnel bribes for overpriced contracts. More indictments are expected.

Federal police, similar to the FBI in the United States, and prosecutors are increasingly relying on international cooperation in the investigation, Marena said. That may prolong the probe but could also help unveil evidence of graft in projects abroad as well as more hidden bank accounts, she said.

The focus is also expected to expand as soon as February to politicians who may have received bribes, when the Supreme Court in Brasilia could open the case.

President Rousseff was chairwoman of Petrobras' board of directors from 2003 to 2010, when much of the alleged graft took place, but she says she had no involvement in the scheme.

(Additional reporting by Jeb Blount; Editing by Brad Haynes and Bernard Orr)