However, FA chiefs said it "was highly unlikely" a points deduction at the start of next season could be enforced.
Leeds are three points adrift of safety, with one match to play.
And with a goal difference far worse than Hull's, only a minor miracle on the last day of the season will keep the Whites in the Championship.
FA spokesman Mark Hooper said: "In the first instance it's a criminal offence to encroach on the pitch and we will be liaising with the police on that matter.
"We will be making various enquiries. We will wait to see the reports from our crowd observers and the referee's match report and will also be seeking observations from the club as to what happened.
"It's too early to say what action the FA would find appropriate but it would be very unusual for that to be a points deduction.
"It's more likely any individual found guilty of an offence will be banned and we would make sure the club is taking the right steps to ensure it doesn't happen again."