Je.s.us
Very scarey. There are no words to try to make this better, that is just a shocking situation.
I strongly suggest using an agency, asking them to check references, then you choose two yourself and call.
Hire someone and have them there on a trial for at least two weekends while you are there - so you can try to get a sense of them.
Ideally in the first week, have someone there with them that you know i.e. your MIL, or a friend or someone, who can also give you a sense of how they are.
Definitely have an ID, definitely do a police check - can be done with thier written consent by any police station. (however this will only show a previous history of convictions and is not going to predict future behaviour.)
At the end of the day, we do leave ourselves open when we invite anyone new into our homes.
Fortunately these stories are the minority, but they are good to know they exist, so the rest of us can be more alert.
Thanks for reminding us that things can go horribly wrong, and we need to be on the look out/ever vigilant.