An elderly woman had a narrow escape when a marine flare blasted through her window.
The 12-inch long missile, let off close to her Brightlingsea home, landed under a bed and filled the room with dense, grey smoke.
Fortunately, 84-year-old Lilian Cook was sleeping in another bedroom at the time and, although shocked by the incident, was not hurt.
The device also failed to function properly and therefore did not burn as fiercely as normal.
Police contacted Colchester's Army bomb disposal team for expert advice and discovered it was a marine flare, normally shot into the air as a distress signal.
A police spokeswoman said: "It was clearly discharged close to the victim's home and fortunately it only partially functioned.
"It goes without saying that these devices are extremely dangerous and this incident could have led to serious injury or worse had the item not partially functioned."
Supt Mick Hicks, who is heading the investigation, said: "Fortunately it did not burn properly. We think the impact of it hitting the window stopped it from operating properly."
Police are concerned similar flares could fall into the wrong hands. They are similar to marine safety flares and there have been no reports of any stolen in the vicinity.
Mrs Cook was too distressed to talk about the incident today.
Anyone with information should contact Brightlingsea police on 01206 302515 or Clacton police on 01255 221312, or Crimestoppers on 0800 555111
"It is vital that anyone who can help trace where this pyrotechnic may have come from or if any others are circulating in the area, they should contact police immediately," the spokeswoman added.
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