Two masked men who were "armed with a firearm" fled from outside a prison after cars were set on fire nearby.
Police said the men made off in a vehicle after nine cars were set ablaze outside HMP Birmingham, which is in the Winson Green area of the city.
The BBC understands the masked men were disturbed by G4S security staff, who spotted them spraying liquid on the car roofs in a car park near the jail.
West Midlands Fire Service said they were called at 13:24 BST on Tuesday.
It said two fire engine crews were sent to tackle the blaze, which was in a car park used by prison staff, and crews had left the scene at about 16:00.
Police said they received reports the men were armed with a firearm, but said no injuries had been reported.
It is understood the men were challenged before one of them pulled out a weapon similar to a handgun, forcing the G4S staff to back away.
The cars were then set alight, with a fire spreading along a row of vehicles, before the masked men escaped through a hole in the fence they had made with an angle grinder.
One witness described the blaze as a "firebomb" with car tyres popping and windscreens shattering.
A company next to the car park said their CCTV captured a group of three men jumping from a Volkswagen Golf, before making their escape five minutes later.
Det Con James Farrell said prison staff had been left "alarmed" by the incident and the motive for the attack was "unclear".
"Firearms officers were at the prison within minutes, but the offenders had already fled," he added.
"The motive for the attack is unclear, but we are trawling CCTV to trace the car which made off from the scene."
HMP Birmingham director Rob Kellett said: "Nobody has been injured. West Midlands Police and the fire service are in attendance, and a fire has been extinguished."