I'd get a second opinion from the police. Call it in again, different time of day or opposite day of the week so you don't get the same officers, but this time don't mention what the neighbor said about these dirtbags claiming to have bought it, that will be a reason to view it as a civil matter. I'd call and say that your house has been vacant for a few weeks, you went to check on it because you're ready to put a tenant in, and the locks have been changed and you suspect there are transients inside, and you want the police to be there when you go inside just in-case. Tell them that you want to press charges for trespassing if people are inside. Be prepared to break in or give the police permission to break in. If the police give you a hard time ask for a supervisor. Try to paint the situation as a crime, not a civil matter. Words matter, if the cops view it as transients trespassing rather than a civil matter they're more likely to act. Also, if the cops do go in and talk to the people you will have their names for legal action. I'm a cop and this is what I would do.
Plan B: see if you know anyone who has connections to a local motorcycle "club", ie gang. See if they want to earn some extra money gently persuading these criminals that its not worthwhile to remain in your house. I read a story a few years ago about squatters who took up residence in a disabled veterans house, and a local MC club paid them a visit and got the issue resolved very quickly!
Additionally, in the event that you get inside with or without police, be prepared to change locks, add an alarm, or have a friend stay until you can. Good luck, and sorry you're dealing with this headache.
