Ioannis Sgouromitis/ TWITTER
Police have closed to Tower Gateway Bridge
The woman parked her vehicle near the Tower Bridge Gateway, near Raley, in California.
Wires are connected to the cars gas tank, according to reports from the scene.
According to one witness, the woman inside the car was holding a sign which read "I want a Fed Public Defender right now” and was holding a plastic bag full of tennis balls with wires sticking out of them
The woman blocked the Tower Bridge in West Sacramento using her vehicle at around 10.20pm GMT (2.20pm local time) and was not arrested by police until 11.50pm GMT (4.20pm local time).
Police are now working to establish whether the threat was legitimate.
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The Tower Bridge and adjacent roads remain closed. Live pictures from the scene do not appear to show emergency services near the vehicle.
Bomb disposal squads and armed police appear to be a safe distance the car.
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A robot appears to be searching the vehicle as the driver door of the vehicle remains ajar.