Police say they believe the massive blaze at an indoor paintball centre in St Helens was started deliberately.
Detectives are treating the Borough Road blaze as a suspected malicious ignition.
The fire started shortly after 5.15pm on Monday.
Initially it was feared two youths were trapped inside the building but this proved to be unfounded.
A police spokesman said: “A joint investigation with Merseyside Fire and Rescue Service was launched and the cause of the fire is currently being treated as suspected malicious ignition.
“House-to-house and forensic enquiries are ongoing in the local area.
“Detectives are also appealing for anyone who was in the vicinity of the building who may have seen anyone acting suspiciously to get in touch.
“Officers are particularly keen to speak to two boys, aged around 14, who witnesses saw in the area around the time of the fire.
“Anyone with information is asked to contact Merseyside Police on 0151 777 6041 or Crimestoppers anonymously on 0800 555 111.”