Celtic supporters have been involved in clashes with stewards during their Europa League game against AIK in Stockholm on Thursday.
Stewards were attempting to find a Celtic fan who had thrown a flare that nearly struck a steward, according to Swedish police. They have told Sky Sports News they have identified the person and will now try to apprehend him.
One Celtic fan was arrested for jumping over the fence and invading the pitch, but was released without charge and given a warning.
Police are also searching for a second man who punched a steward when they tried to apprehend the man who threw the flare. Police are in the process of identifying the individual and are confident they will be able to arrest him as there is good CCTV footage of the incident.
The police were not prepared to release details of nationalities but confirmed all three people were ticket holders in the away end.
A police spokesperson said: "There was not generally a big problem. On the whole, Celtic fans behaved very well".
Before the game - which Celtic won 4-1 to reach the group stages - 10 Hoops fans were taken into custody for being drunk.
Police say most of those detained have now been released.
There was also trouble during Rangers' game against Legia Warsaw at Ibrox on Thursday night, where the game was temporarily halted due to smoke from flares set off by the Polish club's fans clouding the pitch.
Legia fans also unveiled a huge banner of Pope John Paul II above a sign which read "be not afraid", which came after a section of Ibrox was closed because of sectarian chanting at a previous Rangers home game in Europe.
UEFA ordered 3,000 seats to be closed off after Gers fans were heard singing anti-Catholic songs against St Joseph's of Gibraltar.
The Glasgow club also face more UEFA action on Friday after being charged with sectarian chanting following the first leg in the Polish capital.
Former Celtic goalkeeper Artur Boruc was among the visiting fans for the second leg of the play-off and was seen in the heart of the away end.
The 39-year-old once wore a T-shirt with a photo of John Paul II at Ibrox under his Celtic jersey which he unveiled after full-time. Boruc spent five years with Rangers' Old Firm rivals from 2005.