The investigation into suspected £5m income tax fraud in football is focusing on payments relating to five players’ transfers to Newcastle United, including those of Moussa Sissoko, Papiss Cissé, Demba Ba and Sylvain Marveaux, according to sources close to the investigation. HM Revenue and Customs, which conducted an extensive raid on Newcastle’s and West Ham United’s offices on Wednesday morning, is understood to be chiefly investigating the payments made to agents, and whether £5m income tax and national insurance was fraudulently evaded.

The Newcastle managing director, Lee Charnley, who was arrested in a dawn raid at his house on Wednesday by six officers, and questioned during the day, is thought to have been told by HMRC officials that he is not personally under suspicion, and he was released without charge. Charnley was the Newcastle club secretary, not a director or in an executive position, when the deals were concluded. Ba signed for Newcastle from West Ham in June 2011, Marveaux from Rennes the following month; Cissé joined from Freiburg in January 2012, and Sissoko, for whom Newcastle were paid £30m by Tottenham Hotspur when he left St James’ Park last summer, was signed from the Ligue 1 club Toulouse in January 2013.

The French prosecuting authority, the Parquet National Financier, said on Wednesday that it had arrested four people in connection with a request for assistance from HMRC; so far only Marveaux’s name has been confirmed.

At both St James’ Park and West Ham’s London Stadium, around 50 HMRC officers spent hours taking extensive documentation and computers away, understood to contain all of both clubs’ transfer dealings and files on players going back years, not just relating to the five Newcastle deals under investigation. Other premises were also searched, including Newcastle’s Darsley Park training ground and Little Heath, a set of junior West Ham academy pitches.

No member of the West Ham staff was arrested on Wednesday, nor interviewed, club sources said, and it was not made clear whether any West Ham signings are specifically under investigation. Chelsea, to whom Ba was transferred in January 2013, confirmed that they were asked for information by HMRC on Wednesday, indicating that the request was part of the wider investigation and that no allegations were made against the club.

At both Newcastle and West Ham there has been some bemusement among staff at the wide-ranging and dramatic nature of the raids, the removal of computers and the amount of paperwork and electronic information taken away. Given Charnley’s position at the time of the deals and the fact that he was released without charge on the same day, there is also a feeling within St James’ Park that the 5.40am arrest at his family home was heavy-handed.

HMRC has a policy of not commenting on individual tax affairs and declined to respond to questions about its investigation. On Wednesday the tax authority said in a statement that 180 officers had been deployed and that “several men” had been arrested, in the UK and France.