Agents from the Las Vegas office of the U.S. Federal Bureau of Investigation have been in Manila over the past week working with the National Bureau of Investigation (NBI), the local agency tasked with looking into the payments as a potential bribery case, the sources said.

Philippine President Benigno Aquino had ordered an official investigation in November to see if the payments were made in relation to the $2 billion casino resort being developed by Universal on Manila Bay.

The investigation has focused on $40 million transferred to Rodolfo Soriano, a close confidante to the former head of the Philippine gaming authority, during the same time Universal sought and won key concessions for its project in 2010.

The FBI had previously questioned people about the payments in the United States. Its involvement in the Philippines marks a widening of its investigation. The NBI's own probe has been slow-moving, according to one of the sources.

Reuters was the first to report on the money transfers to Soriano in mid-November. Universal, majority owned by founder Kazuo Okada and his son through a family trust, has said it carried out its business in the Philippines lawfully. It has also announced the filing of a defamation suit against Reuters in Tokyo for its reporting on the matter.

Universal spokesman Nobuyuki Horiuchi declined to comment on the FBI's involvement in the case.

Officials at the Las Vegas FBI office declined to comment. No one at the NBI could be immediately reached for comment.

The NBI plans to submit an interim progress report next week and ask for more time to complete its investigation, one of the sources said.

All of the sources spoke on condition of anonymity because the investigation is still ongoing.

The payments to Soriano have also been the subject of two congressional hearings in the Philippines and are being probed by the Nevada Gaming Control Board, which regulates Universal operations in the United States.

NBI agents have escorted their FBI counterparts to various locations including the properties of Soriano and Efraim Genuino, the former head of the Philippine Amusement and Gaming Corporation (PAGCOR), one of the sources said.

Soriano and Genuino could not be reached for comment.

Universal broke ground on the project in January 2012, but PAGCOR under its new chairman has threatened to strip the Japanese company of its license if evidence of bribery is found.

Universal has filed suit in Tokyo against three former employees, claiming they sent $15 million of the total $40 million in transfers without authorization. In rebuttals filed with the court, the employees have either refuted the company's allegations or asked for the disclosure of more information.

Earlier this week Universal announced that it had set up a panel of experts without ties to the company to independently investigate the matter.

(Additional reporting by Nathan Layne and Kevin Krolicki in Tokyo and Joseph Menn in San Francisco; Editing by Michael Perry)

• A Permanent Economic Emergency