They're only squatters if they got in while a window or a door was open. If all was locked up, then it's breaking and entering, thus have no right to stay there.
Hard to prove point number 1 though.
I hope you get this resolved before any damage is incurred.
I manage a 5 unit apartment building in Sherman Oaks, have had a couple move into one of the units claiming to have sublet it... spoke with the owner, who talked to the tenant, that said she didn't sublet it. Now the owner wants me to deal with the people she is calling squatters. In an another life, I collected gambling debts... I've not one problem with moving people out, I do not want to go back to that...
No worries. Simply solution. Go to nearest quickie mart, buy a 40 ouncer. Knock on door, when they answer toss it into parking lot. When they dive after it, enter apartment, close and lock door behind you. Problem solved.
If you are feeling exceedingly generous you can toss their crap out the window after them - I probably wouldn't bother, my guess is most of the furniture is made of old milk crates, 2x4's and maybe an odd cinder block or two.
Simple. Go and talk to the "squatters" and ask to see documentation that shows they are subletting the place. Explain to them that without documentation you will have to go to the police and charge them with trespassing.
By the way, how did they get keys if they were not subletting?
California law is downright bizarre when it comes to stuff like this, even if they have no legal right to be there and the rental agreement clearly states that the tenant is not allowed to sublet. Once they've been there for a certain amount of time getting rid of them can be a real hassle.