In a press conference on Thursday, Mr Belmar said the shots were fired from about 125yds (114m) away, but he did not specify from which direction.
He also said he would "have to imagine" that some protesters "were among the shooters". Detectives were still investigating who was responsible, and no-one had been arrested.
But at about midnight at least three shots were fired as the crowd of protesters was starting to break up, Mr Belmar said.
How Ferguson unrest spread
US police said the injuries of the two police officers were not life threatening
Demonstrators gathered outside the Ferguson police department late on Wednesday
Two US police officers have been shot in Ferguson, a Missouri town hit by riots over the killing of an unarmed black teenager last year.
Eyewitness Keith Rose: "I saw one officer covered in blood - I saw officers running to the aid of that officer"
Both suffered "very serious gunshot injuries" but were conscious, he said.
However, Brown's shooting and the riots that followed spurred a federal investigation. It found overwhelming racial bias in the town's policing practices, though Darren Wilson was cleared of civil rights violations.
In the hours after the policemen were shot, use of the hashtag #BlueLivesMatter spiked on Twitter, driven by self-identified supporters of gun rights and other conservative causes, as well as supporters of the police.
"We were very close to having what happened in New York last year," Mr Belmar said, referring to two police officers shot and killed while on duty. He said he thought it was a "miracle" that such an incident had not occurred during earlier protests in Ferguson.
They were shot during a demonstration after the resignation of Ferguson's police chief, which followed a report alleging racial bias in his department.