West Ham's 3-1 defeat by Tottenham at White Hart Lane on November 25 was marred by travelling supporters mocking the stabbing of a Spurs fan in Rome. A group of away fans was also heard hissing, mimicking the sound of Jews being gassed during the Second World War, and chanting the name of Adolf Hitler.
Millwall were investigated after TV footage showed Leeds forward El-Hadji Diouf being racially abused during a match at The Den on November 18 as part of an expose by Sky Sports News into racism, of which 56 examples were identified at the Championship club.
Both matters were also the subject of separate police investigations concerning the use of racist and/or offensive language and/or gestures.
The FA completed its enquiries once the police investigations - which resulted in arrests being made and charges issued against supporters of both clubs - had been concluded.
In clearing West Ham, the FA acknowledged the work undertaken by the club in the immediate aftermath of the match, which included one of the individuals who was arrested - a season ticket holder - being given a lifetime ban.
A statement issued on the Hammers' official website read: "The club works extremely hard to address all forms of discriminatory behaviour, wherever they occur, and operates a strict zero tolerance policy towards anyone who behaves inappropriately when following the team."
For the incidents of racism at Millwall, the FA noted that following separate police investigations, the individuals caught on camera making offensive/racist remarks were identified, arrested and charged with the club also imposing bans.