The governor of Missouri declared a state of emergency on Monday, authorising the national guard to assist the policing of protests expected in Ferguson after it is announced whether a white police officer who shot dead an unarmed black 18-year-old will face charges.

Jay Nixon signed an executive order on Monday activating the national guard to support police “during any period of unrest that might occur following the grand jury’s decision concerning the investigation into the death of Michael Brown”.

Nixon had previously said he was ready to order national guard troops back into the St Louis suburb whenever necessary. In August, the national guard protected a makeshift police base at a shopping mall during nights of clashes between officers and protesters following Brown’s death in a residential side-street.

The governor said in a statement on Monday that the soldiers would help “maintain peace and protect those exercising their right to free speech” in the event of demonstrations following the grand jury’s announcement, which is expected in the coming days.

“The national guard is well suited to provide security at command posts, fire stations and other locations as well as perform other functions that will free up law enforcement officers to remain focused on community policing and protecting constitutional rights,” he said.

National guard troops in Ferguson would again be commanded by brigadier general Gregory Mason, a veteran of the Missouri state highway patrol, which will police potential unrest alongside the St Louis metropolitan police and St Louis county police.

Statements welcoming the national guard from chiefs of those forces, which represent a unified police command, were attached to the statement from Nixon, a Democrat who before the crisis in Ferguson was viewed as a possible vice-presidential candidate for 2016.

The grand jury has been meeting for about three months to consider evidence on the shooting of Brown by officer Darren Wilson on 9 August. The officer shot Brown repeatedly after an altercation that followed Wilson stopping him and a friend for jaywalking.

Several witnesses said afterwards that Brown was shot after surrendering. Police say that he assaulted Wilson. The officer has reportedly testified to the grand jury that after fleeing, Brown turned and again posed him a physical threat.

A series of leaks from federal and regional officials has led to a widespread assumption among protesters that Wilson, 28, will not be indicted for the shooting. A series of actions is planned in response around the St Louis region and the US.

While the grand jurors have considered material compiled by county investigators, the Federal Bureau of Investigation and US department of justice have been conducting a parallel inquiry for possible federal civil rights charges against Wilson. These, too, are considered unlikely.