Arsenal have identified a man they suspect of throwing a bottle at Dele Alli during Wednesday's 2-0 defeat to Tottenham and are working with the Metropolitan Police to find him.
The Gunners also apologised to the 22-year-old England midfielder, who was hit by the bottle but not hurt, and to Tottenham, saying the incident had "embarrassed" the club and that "behaviour of this type has no place at Emirates Stadium".
Arsenal promised that the individual concerned "will receive a lengthy club ban and their details will be passed to the police to commence legal proceedings". Spurs boss Mauricio Pochettino praised the way Alli reacted to the incident and said he did not feel Arsenal should be punished for the actions of one individual.
The club statement said that the suspect left the stadium immediately after throwing the bottle at Alli in the 73rd minute of the Carabao Cup quarter-final tie but an image of him has been found by analysing CCTV footage of the crowd.
"We are liaising with the Metropolitan Police and investigations continue in order to apprehend the culprit," the club added.
In a separate incident from a troubled night, the Independent reported on Thursday evening that Arsenal were investigating allegations of anti-Semitic behaviour made against some of their supporters.
A statement to the newspaper said Arsenal were "aware" of the claims and encouraged anyone who witnessed such behaviour to report it.
With regard to the Alli incident, earlier on Thursday, a spokesperson for the Met confirmed they were working with the club "to identify the person responsible" but there has been "no arrest in relation to this specific incident".
The spokesperson added, though, that seven people were arrested for a variety of incidents before, during and after the match which included possession of drugs, public order and assault on police.
After the game, Alli, who scored his side's second goal with a delightful lob, told Sky Sports: "It is what it is – it made the goal a bit sweeter and the win."
And his immediate reaction was to turn to the home fans and make a '2-0' gesture. His team's victory earned them a semi-final tie against Chelsea.
Pochettino praised Alli for the control he showed during the incident. He told a press conference to preview Sunday's game at Everton: "It was an amazing reaction.
"In another country the player was going to be down on the pitch, Dele behaved really well, Arsenal should be grateful to him because the player could have gone down and created a massive problem.
"I think the behaviour was top from Dele, sometimes he is criticised but he was very mature."
Pochettino added that he did not think Arsenal should be punished.
He said: "It's not fair for Arsenal, the club, the fans to pay for one...what word can I use? I don't want to be wrong but I think everyone realises! You cannot pay (the price) for one.
"One person cannot create a mess in a club like Arsenal. Arsenal is going to take a big decision, with responsibility to fix. People have to behave and celebrate and support your team but in the right way.
"It's such a dangerous thing, if the bottle hit him on his head or his eyes we're then talking about a very bad thing. There's no point when you go to enjoy a game and that happens."
The Football Association has confirmed to Press Association Sport that it is also investigating the matter but it is up to the police and club to identify the culprit first and make sure they face the appropriate sanction.
Former Gunners striker Ian Wright believes Arsenal must send a strong message that hooliganism will not be tolerated.
"I've got to roundly condemn the person who chose to throw something at Dele Alli," said Wright via his YouTube channel.
"I know Dele can be a bit of a pantomime villain but it's no excuse. I did exactly the same to the Spurs fans and I don't ever remember having anything thrown at me.
"Whatever the club is doing they should fine that person, ban them, ban them for life. We don't want that attached to our club."
A Spurs fan was arrested for throwing a banana skin at Pierre Emerick-Aubameyang when the two teams met in the Premier League at the start of the month.
He was handed a four-year football banning order on Wednesday after pleading guilty to throwing a missile.
Separate trouble between the players during that game has led to Arsenal being fined £45,000 and Tottenham £50,000, the FA announced.