A federal judge has ordered the Federal Bureau of Investigation to give her a better explanation for its refusal to turn over information to a student researching an alleged plot to assassinate "Occupy" protest leaders in Houston.

The ruling stems from a lawsuit brought by a Massachusetts Institute of Technology graduate student who is seeking records from the FBI related to a Houston spin-off of the 2011 Occupy Wall Street protests and an alleged sniper plot. The student claims that the heavily redacted responses...