Police in Portugal are planning to question Manchester United about the club's €9m (£7.2m) signing of the striker Bébé from the Portuguese club Vitória Guimarães, as part of their criminal investigation into the deal.

The judicial police national unit for combating corruption, part of the Justice Ministry based in Lisbon, is investigating the 11 August 2010 transfer, by which United paid that fee for a player whose only competitive experience was a single season in the Portuguese third division.

Bébé's agent, appointed days before he moved to United, was Jorge Mendes, also agent to Cristiano Ronaldo, Nani and Anderson, who moved from Portuguese clubs to Old Trafford in previous seasons. Of the €9m United paid to Vitória for Bébé, Mendes was paid 40%, €3.6m (£2.89m). It was reported in Vitória's subsequent general meeting that Mendes's €3.6m comprised a 10% agent's commission, €900,000, and the further €2.7m because Mendes had also just acquired 30% of Bébé's "economic rights", part-ownership of the player.

Sir Alex Ferguson said at the time that Bébé was the only player he had signed in his long managerial career without having watched him at all first, even on video. United said Bébé had been recommended by Carlos Queiroz, coach of the Portugal national team, formerly Ferguson's assistant at Old Trafford.

On 10 April, the police anti-corruption unit in Lisbon wrote to Bébé's former agent, Gonçalo Reis, asking him to attend at their headquarters and as a witness to provide his account of what happened in the deal. The letter informed Reis that a processo-crime – literally, criminal proceedings – "relate to the transfer of the professional football player Tiago Manuel Dias Correia (known as 'Bébé') from Vit. Guimarães to Manchester United (England)."

The police unit declined to officially confirm to the Guardian the scope of their inquiries or what precise aspects of the transfer they are investigating. Reis has complained formally to the Portuguese Football Federation that Mendes improperly poached Bébé from him before very quickly sealing the United move, in breach of Fifa's regulations governing the conduct of agents.

Reis told the Guardian that on 23 April he was interviewed for around three hours by a police inspector, José Cunha Ribeiro, at the anti-corruption unit's offices. Reis said the police asked him in detail about every aspect of the transfer, and told him they plan to ask United for their version of the deal. Subsequently Reis emailed several documents to the inspector, including his contract to represent Bébé.

A spokesman for United said: "Clearly if the police ask us for information, we will co-operate with them. Nobody is suggesting that we have done anything wrong."

Mendes and his agency Gestifute did not respond to the Guardian's questions about the police investigation. They have previously denied poaching Bébé from Reis. In his complaint to the PFF, Reis claims he had an exclusive contract to represent Bébé as the player's agent for two years from 25 August 2009. Bébé, who had a poor, deprived childhood, his parents having abandoned him, was still living in a care home when he played for Estrela da Amadora, in Portugal's semi-professional third division, in 2009-10.

He played one full season as a striker for Estrela, then after they failed to pay him because the club was in financial difficulties, Reis negotiated the termination of the player's contract and Bébé became a free agent. He moved to Vitória, a first division club, in June 2010, with Reis brokering the terms of the contract. For a time Bébé lived in Reis's house as he came to terms with making his way into professional football. He played in just six pre-season friendlies for Vitória when a story appeared in Marca, the Spanish football newspaper, that Real Madrid, managed by José Mourinho, a client of Mendes, were suddenly interested in signing him. United have always said they then moved rapidly to sign him, on the recommendation of Queiroz, who is also represented by Mendes.

According to Reis's official complaint, on 9 August, 2010 he received a letter from Bébé, dated 5 August, in which Bébé sacked Reis as his agent. Two days later, on 11 August, Bébé was transferred to United, who paid €9m to Guimarães for him, and gave the player a three-year full professional contract on wages Reis believes are €63,000 a month, net of tax. It emerged subsequently that Mendes had become the agent and owner of 30% of Bébé's economic rights, too, and made €3.6m from the United deal.

At United in 2010-11, Bébé was selected to start two matches in the Carling Cup and one in the FA Cup – the fifth round 1-0 victory over Crawley Town – and made four further appearances as a substitute. He scored when he came off the bench against Bursaspor in a Champions League game in Turkey in November 2010. After a year, United sent him on loan to the Turkish club Besiktas, which has several Portuguese players on the books. He has been injured for much of the time and although still contracted to United, spent much of his rehabilitation back in Portugal.