LONDON (AP) -Chelsea lodged a formal complaint to the Football Association on Wednesday accusing a referee of abusing midfielder John Obi Mikel, but dropped allegations he insulted another player.
The Premier League leaders accused referee Mark Clattenburg of directing "inappropriate language" against two players in Sunday's Premier League match against Manchester United.
Players' union chief executive Gordon Taylor said only one of Clattenburg's comments was allegedly of a "racial" nature.
But after interviewing players and staff as part of an internal investigation, Chelsea decided there was only a strong case to show Mikel was insulted.
Juan Mata was initially reported to have also been a target of Clattenburg's verbal abuse, but Chelsea found there was "not sufficient evidence to support a second claim."
London police are also investigating the matter after the Society of Black Lawyers asked officers to look into whether a "racially aggravated offence has occurred" at Stamford Bridge.
Chelsea launched a "thorough investigation" led by external lawyers after lodging an initial complaint to the FA immediately after the match.
"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 the League Cup match against United at Stamford Bridge.
Chelsea officials first became aware of the allegations in the dressing room after the acrimonious 3-2 loss to Manchester United in which two Blues players were sent off.
"Board members began establishing the details of the allegations, speaking to all relevant players and staff," Chelsea said. "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.
"We will cooperate fully with The FA and the police as they each investigate the matter," the European champions said.
Chelsea captain John Terry is currently serving a four-match ban for racially abusing an opponent in a league match last October.
Rob Harris can be reached at http://twitter.com/RobHarris