ST. LOUIS—Missouri Gov. Jay Nixon called for “restraint” Monday as a grand jury reached its decision on whether to indict a white police officer in the summer shooting death of an unarmed black teenager, setting off a national debate on race, justice and policing. (Latest:Ferguson police officer not charged in black teen’s shooting).

A spokeswoman for St. Louis County said the 12-member grand jury had finished its work. A spokesman for the St. Louis County prosecutor said late Monday that an announcement of the grand-jury decision was coming that night on whether to indict Ferguson police officer Darren Wilson in the shooting death of Michael Brown after an altercation between the two. Nine of the 12 members were needed for a vote to indict.

Many businesses boarded up, and schools were closed Tuesday in anticipation of the decision. Hundreds of people already started protesting in recent days as word of a possible decision leaked out. The death of Mr. Brown brought weeks of sometimes violent protests in Ferguson and other parts of greater St. Louis as police responded with tear gas, riot gear and military-style vehicles.

“Our shared hope and expectation is that regardless of the decision, people on all sides will show tolerance, mutual respect and restraint,” Mr. Nixon said Monday evening.

The governor said people in the community remained on edge. The state enlisted the help of churches and counselors, while law-enforcement officials worked to communicate with protest leaders to ensure people can protest, while avoiding violence or property damage. National Guard troops were to be used to secure firehouses, utility substations and other critical structures to free up police, he said.