A boat was intentionally cut from its moorings before it collided with a bridge in Great Yarmouth, police believe.
At around 7.30pm on Monday (January 23), Haven Bridge was closed to traffic after a boat collided with its side, damaging a navigational sign.
Norfolk Constabulary say they believe people entered a secure compound in the town to get to the boat.
It is believed the offenders deliberately cut the vessel from its moorings.
A knife was found inside the secured area and has been taken for forensic testing.
Police say there are currently working on leads and looking at CCTV footage.
The vessel is believed to have been for use in the offshore energy sector, possibly to ferry workers to wind farms.
Norfolk Constabulary said they believe the amount of damage caused to the boat would be a "significant amount".
Haven Bridge was reopen to traffic by Tuesday morning.
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