I think Fincen should be liable since after all MtGox was licensed by FinCen and Fincen got 5 million from Gox which was really their customers money, so to some degree FinCen is liable since they got involved with Gox. Only a good lawyer can answer to what degree they are liable, I am not a lawyer.
MtGox wasn't "licensed by FinCEN". FinCEN doesn't license anyone. It's a Department of the US Treasury which is responsible for collecting financial intelligence. Licensed financial services are required to register with FinCEN and provide it with certain information in relation to transactions conducted by their business.
The MtGox funds which were seized by the government were seized by the Department of Homeland Security. More specifically, it was the US Secret Service (a department within DHS) which obtained the warrants to seize the funds.