More: Grand juries decline to indict, known as a "no true bill," in only a small fraction of federal cases. To secure an indictment prosecutors need only show probable cause a crime was committed. That's much lower than the standard in criminal trials. A grand jury declining to indict three times on the same case is a warning the evidence may not stand up at trial, according to attorney Christopher Macchiaroli, a partner at Silverman Thompson Slutkin White who previously served as an assistant U.S. attorney in the federal prosecutor's office in D.C.

"Seeking an indictment for a third time is extremely rare and usually only reserved for the most serious of crimes," Macchiaroli said. "If a governmental entity cannot convince a super majority of grand jurors that there is a fair probability that a crime was committed, it is virtually impossible to believe that twelve jurors in the same relevant jurisdiction could unanimously at a future date find guilt beyond a reasonable doubt, the highest standard of proof under the law."

Reid was arrested in July for allegedly resisting attempts to restrain her after she refused to back away from ICE officers who were conducting arrests outside the D.C. Jail. In the process, they said, an FBI agent received scrapes to the back of her hand.