It's a bit of both. Police aren't going to arrest you unless they're pretty sure the charges are going to stick, but once you're in custody, they are... well,

It's a bit of both. Police aren't going to arrest you unless they're pretty sure the charges are going to stick, but once you're in custody, they are... well, let's just say "highly motivated" in getting you to sign a confession, and they have a great deal of leeway that is unavailable in the US.