I imagine this is just a courtesy; if they decline to answer, then the committee can issue a subpoena and compel them to.
Here's a nice white paper on Congress's subpoena power: http://www.mayerbrown.com/files/Publication/ec1203b2-a787-44...
Here's a nice white paper on Congress's subpoena power: http://www.mayerbrown.com/files/Publication/ec1203b2-a787-44...
"The Enforcement Process. Congressional investigations often begin informally, with the interested committee or subcommittee first seeking information on a voluntary basis (i.e., by sending a letter request or asking for an informal interview), rather than by issuing compulsory subpoenas. Although there is no legal obligation that a party comply with such a request, it is typically in the responding party’s best interest to do so"