同学小薇: A follow-up to your question

When I was reading The Fifth Witness by Michael Connelly yesterday, I came across this line similar to what we were discussing:

P.43  "I didn't think so.  So all I'll say then is be careful."

The author did not use "to be careful".

I still don't have a clear explanation to that, but I tentatively believe, by dropping "to", it would give the statement a warning, or an admonishing or a demanding undertone.