Police said the man was spotted yesterday [Tuesday, May 10] in Wakehurst Drive by a member of the public. He was then sighted later on The Marsden.
Sussex Police are now appealing for anyone with information on the individual to report it to the police
A police spokesperson said: "Just before 4.20pm police responded to a report from a member of the public who had seen a man on a BMX carrying a firearm in Wakehurst Drive, Crawley. He was later seen in The Marsden.
"Officers attended the area and conducted an expansive search but could not locate him.
"The suspect is described as white, slim with brown hair and was wearing a grey hoody and dark jogging bottoms.
"Anyone with information which could help officers with their investigation is asked to report it online or call 101 quoting serial number 892 of 10/05."