Mark Clattenburg is understood to have been among those interviewed by the English FA yesterday as part of continuing investigations into Chelsea's claim that the referee used "inappropriate language" towards John Obi Mikel.
Chelsea submitted a dossier of evidence to the FA, including signed witness statements from players, alleging Clattenburg used a term understood to have been interpreted as racist during the Premier League match against Manchester United on 28 October.
The governing body are now assessing those allegations and are believed to have spoken to Clattenburg, assistant referees Michael McDonough and Simon Long and fourth official Michael Jones.
The interview process is not expected to go beyond today, meaning there should be news on whether the case will go any further early next week.
The full file of evidence from Chelsea was received by the FA at the start of the week and since then the players have also been interviewed.
The case could not be more sensitive following Chelsea skipper John Terry's four-match ban for racist abuse, and the FA is determined to deal with it as quickly as possible - especially following criticism about the length of time, almost a year, it took to deal with Terry.
Clattenburg, meanwhile, has decided not to referee a match for a second consecutive weekend while the investigation is ongoing.
The Metropolitan Police has also launched an investigation following a complaint lodged by the Society for Black Lawyers, based on reports of the incident rather than any evidence.
Manchester United manager Alex Ferguson has had his say, saying it was "unthinkable" that Clattenburg would make such a comment and that he was "convinced" he would not have done so.
Clattenburg, 37, who is on FIFA's shortlist to be a match official at the 2014 World Cup, strenuously denies the accusations against him.