Images shown of concrete blocks thrown on to car in Essex
Images of concrete blocks which were dropped from bridges on to the A12 in Essex, seriously injuring a car passenger, have been released.
Police said the bucket-sized blocks showed the potentially fatal consequences of the "despicable act" on Thursday night.
One woman was seriously hurt and two women were left shaken when blocks went through the windscreens of two cars.
Both attacks are being treated as attempted murder.
A 57-year-old woman, from Harold Hill, was with her husband when a block was dropped from West Hanningfield bridge, near Galleywood, at about 22:00 GMT.
She had to be cut out of the car and is a serious but stable condition in Broomfield Hospital with severe face and chest injuries.
Her husband, aged 56, suffered minor cuts and bruises.
Another block was dropped from Fryerning Lane bridge, near Ingatestone, also on the A12, at 21:20 GMT, smashing the windscreen of a Vauxhall Astra.
A woman aged 48 and her 26-year-old daughter, both from Brentwood, were uninjured.
Police are linking the attacks, which happened about eight miles apart, and are analysing several items recovered from the bridges.
They said the perpetrators would have needed a vehicle to transport the heavy blocks and to get to the remote locations.
Det Ch Insp Keith Davies said: "These particular incidents illustrate the severity of the criminal acts we are dealing with.
"Even a stone thrown from a bridge can be fatal, but when you look at the size of this concrete, it is amazing that the consequences were not far, far worse.
"It appears to be a bollard of some nature so we are appealing to anybody who knows where some may have been removed to contact us."
Detectives are also looking back at more than 30 similar incidents over the past three years for any links to Thursday's attacks.
House-to-house inquiries are on-going and police have revealed one of the blocks had been stolen from a house near Galleywood.
They are keen to receive details of suspicious activity, reports of other incidents or information about the removal of hardcore or building rubble.