Two stewards have been shot during a concert held by rapper-producer Kanye West at the NEC in the West Midlands.
Police are carrying out forensic tests at the NEC
Police said the shootings took place after people who tried to gain entrance without tickets were escorted from the arena near Birmingham on Tuesday.
One steward was still being treated in hospital, where he is in a serious condition after being shot in the face.
Officers are looking for a black man, aged 18 to 22, 5ft 10in tall, slim, with a moustache and plaited hair.
Police have stressed they are not treating it as a gang-related incident.
The man, who had no ticket, entered the foyer and was ejected. He then returned and shot stewards. He was with a black woman.
Officers said between four and eight shots were fired.
A 45-year-old from Solihull was shot in the face, stomach and foot and has undergone one operation and is due to undergo another.
A second security guard has been released from hospital. He may have received a "graze" injury.
One man and a woman have been arrested.
Police were called to the NEC Arena at just after 2200 GMT on Tuesday.
'Vicious and cowardly'
Det Ch Insp Kenny Bell from West Midlands police told a news conference: "We've identified numerous witnesses and I am very interested in the CCTV that we've seized that we're now analysing to try and identify who the gunman was.
"I'd just like to emphasise again I do not treat this as a gang related incident.
"Over 12 thousand people were at the NEC last night - it's only one of them who decided to go out with a loaded firearm and use it in this vicious and cowardly way".
Witness Ellie Cullister was standing outside getting some fresh air and was near the scene of the shooting.
She told BBC WM: "I turned around and saw this group of guys and I heard a gun shot fire and three people run away.
"Everyone crowded round them but the police were soon there and the stewards were telling everyone to back off and get inside.
"You could not see much, but it was really frightening."
Final UK date
The scene of the shooting remains cordoned off for forensic examination.
Detectives have asked for anyone who saw anything unusual at the concert to come forward.
Kanye West - born in Atlanta, Georgia and lately working in Chicago - made a name for himself in the rap world as a producer, and then launched his own recording career.
He was performing the last night of his five-date Touch The Sky tour in the UK.
The NEC Arena, which was open for business again on Wednesday, said it would be reviewing security arrangements.