Teenage boys are being coerced into taking part in violent city car-jackings, police fear.
West Midlands Police spoke out after four teenagers, one aged 13, were arrested after an attack in Solihull in which a stun gun device was used.
There have been a several similar incidents across Birmingham, some in Kings Heath. Two 14-year-old boys were recently charged with robbery.
A driver suffered slight burns to his hands in the latest incident.
He was threatened for his VW Scirocco in Burford Close, Solihull on Tuesday evening.
The car was later found in Broadstone Road, Alum Rock, along with a Vauxhall Vectra thought to have been used by the gang.
Officers found the stun gun device plus the victim's car keys and mobile phone.
Four teenagers, two boys aged 16, a 17-year-old, the 13-year-old and a woman, 30, were detained on suspicion of robbery and released on bail. They are all from Birmingham.
Sgt Carl Grinnell said it was disturbing one of the suspects was aged only 13 and that two 14-year-olds were recently charged over car robberies.
"There is a suspicion young teenage boys are being coerced into taking part in these types of offences," he said.
"I would strongly urge anyone asked to participate in such offences to refuse: these teenagers are jeopardising their entire futures by getting involved and may well find themselves spending time in youth offending institutes."
At least three women were attacked for their cars between 6 and 10 February, in Moseley and Kings Heath, in incidents where stun gun-type devices were used, police said.