Reply #8: The sheriff can't just arrest someone without probable cause.

[JDPriestly]

He probably needed to be able to charge the person with something.

Also, the police may have gone to the house because they were called about something else -- such as a different family member. There have been suggestions that an additional family member was a very angry person. Police are often called regarding matters involving domestic violence, so that is also a possibility.

In an early interview, the sheriff suggested that Loughner had a bit of a record. I believe I read there were charges of graffiti or something like that. We will have to wait and see. Your speculation is unfounded so far.