Arsenal and Uefa have opened separate investigations into the circumstances which led to the Gunners’ 3-1 win over Cologne being overshadowed by crowd trouble.

Kick-off was delayed by an hour as approximately 20,000 away fans descended on Emirates Stadium despite an official allocation of 2,900.

That led to some supporters attempting to force their way into the ground and once the turnstiles reopened, thousands of Cologne fans found their way into home sections of the stadium breaching rules regarding segregation of fans.

Arsenal released a statement this morning which read: “Following last night’s Uefa Europa League match with Cologne we would like to stress that fan safety was always our paramount concern and informed all decisions made.

“We worked in full consultation with police and Uefa officials on the night and in advance of the game and had taken extensive steps in advance to prevent tickets being sold to visiting supporters.

“This included a number of measures such as ensuring no tickets were sold via general sale and that no red memberships purchased after the draw were able to be used to get tickets in the home end for this match. We also worked with our colleagues at Cologne to stop supporters traveling without match tickets.

“The 3,000 tickets issued to Cologne fans were in line with competition rules but it is clear many more visiting fans arrived, causing significant congestion and disturbance outside the stadium before kick-off. Many tickets were sold through touts and this is very disappointing and something we continue to work hard to address.

“We have launched a full review into the circumstances surrounding the game and will ensure any lessons that can be learned are used in the future.”

The Gunners could face a long list of Uefa sanctions relating to Article 16 of their Disciplinary Regulations, entitled ‘Order and security at Uefa competition matches’, which states: “Host clubs are responsible for order and security both inside and around the stadium before during and after matches.

“They are liable for incidents of any kind and may be subject to disciplinary measures and directives unless they can prove that they have not been negligent in any way in the organization of the match.”

Uefa, the Metropolitan Police and both clubs agreed after lengthy consultation that the match would go ahead. Segregation is deemed a club issue with Uefa this morning waiting for their delegate’s report before deciding on the next steps.

An investigation has begun although specific charges were not yet clear. Uefa’s guidelines suggest that issues of spectator control carry a possible 5,000 euro fine, public passageways an 8,000 euro fine and doors and gates a 15,000 euro fine.

Problems cited as ‘admission of spectators’, ‘ticket quotas and prices’ and ‘visiting team and supporters’ are punishable only by written warnings as a first offense.

Cologne will almost certainly face a 1000 euro fine as their supporters set off two flares inside the ground during the first half, in addition to any separate charges relating to the conduct of their fans.

Some Arsenal fans sold their tickets to desperate Cologne supporters keen to see their club’s first European fixture in 25 years. One of the main issues to be discussed is how so many fans were marched to the stadium in proportion to the ticket allocation available.

When contacted this morning, Met Police insisted that was a matter for Arsenal. Met Police also refused to confirm whether a ring of steel around the ground, preventing anyone without a ticket from approaching the stadium, was discussed.

Once thousands of supporters of both clubs were inside the ground, it is understood the police considered the “best and safest” option was to allow the fixture to go ahead, despite the breakdown in segregation, due to the difficulty of dispersing such a large, disgruntled crowd at once.

In addition to the supporters’ issues, Sead Kolasinac may also face an individual charge under Article 15 – misconduct of players and officials – for the specific offence of provoking supporters.

Kolasinac cancelled out Jhon Cordoba’s early goal within five minutes of coming on as a half-time substitute. In celebrating, Kolasinac lifted up his Arsenal kit to reveal a shirt with a slogan which, translated into English, read: “North curve in your city”.

The north curve is the name of the hardcore element of Schalke fans. Schalke is Kolasinac’s former club and although there is no close rivalry with Cologne, the slogan could be seen as incendiary.

It is within Uefa’s remit to ban Kolasinac for one European match on such grounds. Minor scuffles broke out throughout the game with Cologne fans sporadically ejected from the home end as Arsenal gradually took control of proceedings on the pitch through goals from Alexis Sanchez and Hector Bellerin.

Standard Sport spoke to several stewards and stadium security staff who were all surprised the game had been allowed to go ahead.

Although segregation is much more relaxed in the Bundesliga, the sight of thousands of away fans in the home end is a rarity both in Uefa competition and in English football. It is, therefore, something of a relief that only five arrests were made.