Police and AD Police

[Wolfpac]

I'm not sure if this is just in Anime or real life in Japan but I'll post it here anyway

What's the difference between the Police and AD Police???
Is there any difference???
What dose AD stand for (apart from After Death)

[fireguy]

Police are the normal run of the mill cops and AD Police are the specialist cops like SWAT or something similar. Can't think of what the designation AD would stand for?