Uhm, this is extortion and I'd be contacting local and state police.
It might be extortion; the details matter. But if there is any chance the OP is at risk of a fraud prosecution, he or she really needs to consult a Colorado criminal defense attorney before talking to anyone, especially law enforcement. Note that Colorado does not have a state police. It has a state highway patrol and its jurisdiction is limited to enforcement in highway related matters. The proper agency to contact is either the city police or, if the threat was not made in any city, the county sheriff.
There's been no slander as such so far, but instead defamation (if verbal) per se or (if written) libel per se as it relates to "file fraud charges against you" (assertion of criminal behavior).
You do understand that defamation is the term that covers both slander (verbal defamation) and libel (written defamation) right? Thus, asserting that there is no slander but that there is verbal defamation makes no sense.
Also, whether there was any defamation here obviously depends on the exact statement made and whether the statement asserted as fact something that is untrue. There is not enough information here to know whether any defamation occurred.
I'd discuss with police and, if former employer goes to police, a local civil litigation attorney.
Understand that generally complaints to law enforcement have a high degree of protection. A defamation claim for a complaint to the police isn't likely to succeed.
Because CO is one if a minority of states where there are still criminal consequences for defamation per se as it relates to DEAD folks, you can presume that CO is one of the states where defamation/libel per se is still technically a crime if committed against the living. :) (This isn't to say that police or prosecutor would pursue, but ...)
That's incorrect. The Colorado legislature last year repealed its criminal libel statute, effective September 1, 2012. It is no longer possible to be convicted of criminal libel in Colorado for statements made on or after that date.
I suggest that the OP simply ignore what he or she is hearing from the former employer and co-workers. There is no obligation to participate in any deposition or providing any statement for the former employer unless a subpoena is served. If that occurs, consult a Colorado attorney about how to handle it.
The ex-employer is free to make a fraud complaint to the police if he wishes. The OP cannot stop that and trying to interfere with that presents possible criminal problems of its own. The DA won't file charges unless there is some evidence to support the claim. If the OP is contacted by police, again a lawyer should be consulted.
Otherwise, at this stage it is probably not worth contacting the former employer to threaten anything (e.g. defamation lawsuits, whatever). That simply risks adding fuel to the fire, as it were. The ex-boss is going to be primarly focused on his own legal problems — the tax case that is apparently pending against him. Let that remain his focus, rather than focusing his attention on the OP.