The FA has contacted Manchester United and Liverpool to ask why flares were let off at Old Trafford during their Capital One Cup tie on Wednesday.
Four were let off during United's 1-0 win, with one reportedly thrown from the top tier of East Stand, where Liverpool fans were sitting, onto lower tiers in which United supporters were housed.
An FA spokesman told the Daily Telegraph: "The Football Association has contacted Manchester United and Liverpool to seek their observations into the incidents with flares during the Capital One Cup tie.
"The FA, Premier League and Football League are working together on the issue of flares, in conjunction with the Fire Service and St John Ambulance, in an effort to alert supporters to the potential dangers."
The Premier League warned both United and Liverpool of the dangers that flares could pose, adding in a statement: "Flares burn at close to 2,000 degrees, and we have been made aware of several incidents where fans have sustained injuries by igniting them.
"They also generate such large quantities of smoke that they negatively impact on people's enjoyment of the game and can cause panic and breathing difficulties for fans sitting close to them.
"We will be stepping up our work with supporter groups to help fans understand the dangers involved with flares and encourage the minority who are putting others at risk to stop using them."
Greater Manchester Police arrested ten people during the Capital One Cup game.