A top aide to Democratic U.S. Rep.
Gabriela Marquez, district director for Cardenas, reported the subpoena last week as required by House rules. Her letter was read aloud on the House floor Thursday.
The FBI asked Marquez weeks ago during an interview in her home whether Cardenas' congressional employees were performing campaign work on government time, according to unnamed sources cited in Roll Call, which first reported the subpoena Tuesday. If his staffers were doing campaign tasks during congressional work hours, that would violate the law and House rules.
An FBI spokeswoman, citing agency policy, would not comment. Cardenas' spokesman did not immediately respond to a request for comment.
Cardenas, a former member of the Los Angeles City Council, is serving his second term in the House.