Police in Portugal are planning to question Manchester United as part of their investigations into the controversial transfer of Bebe.
The Premier League club signed the Portugal Under-21 forward from Vitoria Guimaraes for £7.4m in August 2010.
An anti-corruption unit is examining the transfer to establish how £3m of the fee was purportedly paid to Jorge Mendes's Gestifute agency.
United say they will cooperate with any police investigation.
Bebe at Old Trafford
Bebe made seven appearances during his first season at Manchester United, with just two of those coming in the Premier League - both from the bench
The review of the Bebe deal is part of a wider police probe into the finances of Guimaraes, who announced profits of £16.5m last year.
United have not been contacted by investigators at this stage and a spokesman stressed that "nobody is suggesting that we have done anything wrong".
Last month, Portuguese police interviewed the player's former agent Goncalo Reis.
Reis was sacked as Bebe's agent six days before United signed a player Sir Alex Ferguson later admitted he had never seen play.
Although police do not suspect Reis of any wrongdoing, he was asked to attend a police station in Lisbon last month to discuss how he came to be replaced by Mendes's agency and his relationship with Bebe.
The police - who are understood to possess copies of documents relating to the transfer - are said to have scheduled a series of hearings with witnesses.
Gestifute insist the transfer was done correctly and entirely in accordance with Fifa rules and regulations.
Bebe spent much of his youth living in a homeless shelter after being abandoned by his parents, prompting Ferguson to describe his move to United as a "fairytale".
Did you know?
In 2009 Bebe was a resident at the Casa do Gaiato shelter in Portugal, an organisation that helps homeless people and uses football as a way of getting them off the streets
The 21-year-old has made little impression at United, starting only three matches, before joining Besiktas on a season-long loan last summer.
His spell in Turkey has been equally difficult, however. He tore an anterior cruciate ligament in his left knee while playing for Portugal Under-21 in Slovakia in August and missed seven and a half months of the season.
Before then, he had played only three times for Besiktas and last month he was suspended from first-team training by the Turkish club after breaking a curfew by visiting a nightclub on the eve of a game against Galatasaray.
Tayfur Havutcu, the coach of the Turkish club, confirmed the player had been disciplined for his behaviour and could even be sent back to Old Trafford. The loan agreement with Besiktas has an option for a permanent £2m move.