Hello,
The best way you already did, message them. If they ignore your messages it's most likely because they are engaged elsewhere.
What I tend to do at this point is either send myself to jail via the courthouse OR check sidechat/ask sidechat for the location of the police and wait in the nearest HQ.
Also, be creative with your self defense story not just "They tried to rob me so I killed them all, can you pardon me?", it might have happened like that sure but most cops wont full out pardon you for such, you'll get the first pardoned and the rest in the form of a half ticket. (Unless it's changed I've not been a cop for some time now)