Many of the elaborate plans for how the grand jury’s decision would be released — including 48-hour notice for the police after the decision — appeared to have been scrapped. The family of Mr. Brown, 18, who was killed by Officer Wilson on Aug. 9, was notified by prosecutors in the afternoon, after some reports had already appeared on television and online. A lawyer for the family expressed frustration that they had not been told sooner.

The lawyer, Benjamin Crump, added that the family would be exploring their legal options now that the grand jury has failed to indict Officer Wilson. “They don’t trust this prosecutor; they never did from the beginning,” Mr. Crump said. “And they are going to try to see if they can do something to get some positive change out of this because they understand this system needs to be changed.”

Since August, Officer Wilson has stayed close to St. Louis, reading news articles and following television coverage of the case, those close to him said. He has made no public statements or appearances. In a private ceremony in October, he married his fiancée, Barbara Spradling, also a Ferguson police officer, court records show. Officer Wilson, who testified before the grand jury for more than four hours, saying he was convinced that his life was in danger, remains on paid administrative leave from the police department, but local officials said they expected that he would resign in the coming days, regardless of the grand jury’s decision.

The Brown family has, by contrast, traveled widely to speak out, including appearing at the BET Hip Hop Awards, meeting with United Nations officials in Geneva and talking with protesters near the spot where Mr. Brown was killed.

Mr. Brown’s father, Michael Brown Sr., handed out turkeys to needy families over the weekend, and he filmed a public service announcement urging calm once the grand jury decision was announced. The parents have been pushing for what supporters have called the Michael Brown Law, which would require officers to wear body cameras.

As the news of the decision spread, school officials were deciding whether to open schools on Tuesday. At least one district canceled after-school and evening activities, and at least four announced they would not hold classes on Tuesday.

All around, there were signs of businesses closing at the prospect of trouble. At least two area malls, including the St. Louis Galleria and the Plaza Frontenac, closed early on Monday evening.

Another investigation, a federal civil rights inquiry into the case, continues, though federal officials have said that the evidence so far does not support such a case against Officer Wilson. A second federal investigation is examining whether the Ferguson police have engaged in a pattern of civil rights violations.