A female passenger was left fighting for life after being struck by a concrete slab hurled from a bridge onto a busy main road.
The woman suffered severe head and chest injuries when the slab smashed through the windscreen of the car at 10.20pm on Thursday.
Police said they were treating the attack, which took place from the West Hanningfield bridge on the A12, near Chelmsford, Essex, as attempted murder.
Mindless: Yobs hunted for attempted murder after concrete slab hurled from bridge
The woman, from Harold Hill, Essex, was rushed to Chelmsford's Broomfield Hospital suffering life-threatening injuries, including multiple fractures, chest and head injuries.
The driver of the vehicle escaped with minor cuts.
Police are linking the incident to a similar attack less than an hour earlier, in which a car was struck by a block of concrete.
The passenger and driver escaped injury but were left shaken and upset by the attack.
At the scene: Police appeal for information from drivers and eyewitnesses
East of England Ambulance service spokesman Gary Sanderson said: 'First and foremost this is a despicable act to carry out.
'We believe the concrete was thrown from a bridge across the carriageway when it collided with the car entering the windscreen and hitting the woman.
'On arrival of the ambulance crews, the woman was found with serious chest and head injuries and following rapid treatment, stabilisation and immobilisation by the paramedics and doctor, she was taken to an awaiting trauma team at Broomfield Hospital.'
An Essex Police spokesman said: 'Detectives are linking both incidents and treating both as attempted murder.
'Both bridges have been taped off while forensic investigations continue.'