A group of Ipswich Town supporters threw flares into the club's training ground, causing the team's session to be briefly halted.
There were 12 flares thrown over a fence as the players prepared for Tuesday's Sky Bet League One match against Northampton at Portman Road.
Signage on the fence caught fire and training was suspended - while a member of the coaching team kicked the flames out - but no one was hurt in the incident, and training resumed around 10 minutes later.
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Suffolk Constabulary have called on anyone with information about the incident to come forward.
A spokesperson said: "Police were called at 10.30am yesterday (Monday 15 February) to reports of a number of people setting off flares on Playford Road in Ipswich.
"Those involved reportedly left the scene on foot, heading towards Rushmere Heath. Police attended and searched the area but found no trace of any suspects.
"Enquiries are ongoing.
"Anyone with any information about this incident or who saw anything at the time is asked to contact the Police."
Ipswich, who are 12th in League One following Tuesday's defeat to Peterborough and finished 11th last season on points-per-game following Championship relegation at the end of 2018/19, reported the incident to the police and an investigation has been launched.
A club spokesperson said: "There was an incident at the training ground this morning where flares were thrown over a perimeter fence adjacent to the players.
"The flares caused signage on the fence to catch alight and in doing so put our staff at risk. The police were called and are investigating the matter. The supporters' group concerned have apologised for their actions."