A woman and her child were lucky to escape unharmed after a brick was hurled at their car from a road bridge.
Charlotte Kenny, 20, from Llanelli, had been travelling along the 60mph stretch to Trostre roundabout when suddenly she heard a loud bang.
Her five-month-old son had been strapped in the back when the incident happened.
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Charlotte said: “I was driving down the 60mph road from Farm Foods roundabout to Trostre at around 3pm when I heard this massive bang.
“A group of kids chucked a brick at my car and if it was an inch further back it could have smashed the window where my five-month-old baby was sat.
“I am so shaken up and angry that kids think it’s acceptable to be throwing bricks off bridges and there will be consequences.
“This could have ended up much worse and I’m just thankful that my baby is safe and was blissfully unaware that anything happened.”
She added: “I was so shaken up as I didn’t see it coming.
“Went over the bridge and it just flew down.
“If it had smashed my windscreen I could have tried to swerve and crashed.”
Police are looking into the incident.
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A Dyfed-Powys Police spokesman said: “Dyfed-Powys Police is investigating reports of criminal damage on the A484 in Llanelli.
“Items were allegedly thrown from a footbridge, causing damage to vehicles travelling on the A484. Enquiries are ongoing.”