"CCTV footage from approximately 47 cameras inside Upton Park is still being examined along with other lines of enquiry.
"Both clubs have cooperated and provided assistance in relation to the enquiry and the FA expects to be in a position to confirm the outcome of the investigation in the coming week."
It was the first time east London rivals West Ham and Millwall had played each other since April 2005 and hundreds of fans were involved in violent scenes reminiscent of the 1980s, when English football was dogged by hooliganism.
Eyewitness reports suggested opposing fans threw bottles and bricks at each other and started small fires as police moved in to try and quell the disturbances.
Inside the stadium, police and stewards battled to remove fans from the pitch throughout the match, which West Ham won in extra time.
After the match, riot police lined the streets outside the ground, roads were closed and a helicopter flew overhead in preparation for further possible clashes as thousands of fans tried to leave the stadium at full time.
Hammers boss Gianfranco Zola said he was "shocked" by the scenes, adding that he had not seen anything like it in seven years as a player in England.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.