Looks like a relevant law might be "18 U.S.C. §1203":
* https://www.law.cornell.edu/uscode/text/18/1203
* https://www.justice.gov/archives/jm/criminal-resource-manual...
This seems to fit American-citizens holding your daughter, with threat to continue doing so, in order to compel you to do something (like sign a contract).
Did charges ever get filed against the kidnappers? Like back in the US?
And was the professor reported to their university?