Federal prosecutors on Tuesday were unable to secure a felony assault indictment against a man who threw a sandwich at a federal agent on the streets of Washington this month, according to two people familiar with the matter.

The remarkable failure by the U.S. attorney’s office in Washington was the second time in recent days that it was unable to persuade grand jurors to bring an indictment in a felony assault case against a federal agent. And it amounted to a sharp rebuke by ordinary citizens against the team of prosecutors who are dealing with the fallout from President Trump’s move to send National Guard troops and federal agents into the city on patrol.

The rejection by grand jurors was particularly noteworthy given the attention paid to the case. Video of the episode went viral on social media, senior officials talked about the case, and the administration posted footage of a large group of heavily armed law enforcement officers going to the apartment of the man, Sean C. Dunn, to arrest him.

It remained unclear if prosecutors planned to try again to obtain an indictment against Mr. Dunn, 37, a former Justice Department paralegal. They could also forgo seeking felony charges and refile his case as a misdemeanor, which does not require an indictment to move forward.

Mr. Dunn was initially charged on Aug. 13 in a criminal complaint accusing him of throwing a submarine sandwich at a Customs and Border Protection officer who was on patrol with other federal agents near the corner of 14th and U Streets in the northwest section of the capital, a popular part of the city filled with bars and restaurants.

Before he threw the sandwich, the complaint asserts, Mr. Dunn stood within inches of the officer, calling him and his colleagues “fascists” and shouting, “I don’t want you in my city!”