LONDON (AP) -- Chelsea sent a file of evidence to England's Football Association on Wednesday alleging referee Mark Clattenburg abused midfielder John Obi Mikel during a match but dropping claims he insulted another player.
The Premier League leaders have only publicly accused Clattenburg of directing "inappropriate language'' at the Nigeria international during Sunday's Premier League match against Manchester United, but the players' union confirmed that the alleged comment was of a "racial nature.''
After lodging an initial complaint to the FA immediately after the acrimonious match, which United won 3-2, Chelsea launched a "thorough investigation'' led by external lawyers.
"Having completed that process we have now followed the correct protocol and lodged a formal complaint with The FA regarding John Mikel Obi,'' Chelsea said in a statement on Wednesday night ahead of a League Cup match against United at Stamford Bridge.
"This is a result of a thorough investigation carried out by the club, and led by outside legal counsel, of all information available to us including interviews with a number of players and staff.''
Those witness statements have now been included in the complaint sent to the FA by the European champions.
Clattenburg is yet to publicly respond to the allegations.
Juan Mata was initially reported to have also been allegedly the target of a comment from Clattenburg, but Chelsea found there was "not sufficient evidence to support a second claim.''
London police are also investigating Sunday's events after the Society of Black Lawyers asked officers to look into whether a "racially aggravated offence has occurred'' at Stamford Bridge.
It is the second major racism case involving Chelsea in the past year.
Chelsea captain John Terry is currently serving a four-match ban for racially abusing an opponent in a league match last October, although he was cleared in a criminal trial of a racially aggravated offense.
Chelsea officials first became aware of the allegations in the dressing room after the match in which two Blues players were sent off and Javier Hernandez appeared to score the winner from an offside position.
"Board members began establishing the details of the allegations, speaking to all relevant players and staff,'' Chelsea said in its first detailed statement on the case. "After those initial interviews it was clear that the matter had to be reported.
"The correct protocol for doing so was to report the incidents to the match delegate, and the club took immediate steps to inform him as soon as he was available. It was not a decision the club took lightly.''
Chelsea complained to English football's governing body rather than the Metropolitan Police, which launched a formal probe on Tuesday.
"We will cooperate fully with The FA and the police as they each investigate the matter,'' Chelsea said.
Clattenburg won't be officiating this weekend after the referees' body said the "intense level of scrutiny would detract from the match and be unfair to the clubs and the supporters of both sides.''
Clattenburg refereed the men's final at the London Olympics in August and is a FIFA candidate to officiate at the 2014 World Cup in Brazil.