Apparently one of the Ferguson, Missouri grand jurors who heard evidence about the killing of Michael Brown by law enforcement wants to talk about the case but can't without violating the law. Thus, this former grand juror with the help of the ACLU has filed a lawsuit (Grand Juror Doe v. Robert McCullough) against the local prosecutor. According to the lawsuit, the grand juror seeks a declaratory judgment that Missouri's laws criminalizing speech by the grand juror about his/her experience is unconstitutional.
Like most states that use the grand jury, Missouri prohibits grand jurors from talking about evidence heard by the grand jury. Historically, grand jurors were prevented from disclosing evidence in order to maintain secrecy of the investigation and to protect the privacy of those who appeared before the grand jury but were never charged with a crime.
Here is the Missouri statute on the topic.
540.320. No grand juror shall disclose any evidence given before the grand jury, nor the name of any witness who appeared before them, except when lawfully required to testify as a witness in relation thereto; nor shall he disclose the fact of any indictment having been found against any person for a felony, not in actual confinement, until the defendant shall have been arrested thereon. Any juror violating the provisions of this section shall be deemed guilty of a class A misdemeanor.