Story highlights Five people arrested, two officers injured, one hit in the face with a rock, police say

Ferguson has been in the national spotlight since Michael Brown was shot in August

Police: Someone apparently set a custard shop on fire after pouring gas around it

Protesters said they were angry that someone burned a Brown memorial earlier Tuesday

Five people were arrested as tensions flared again in Ferguson, Missouri, with people threatening officers, shooting at them, throwing rocks and bottles, and one person tossing a Molotov cocktail at a parking structure, police say.

Two officers were injured, one struck by a rock below the eye, Capt. Ron Johnson of the Missouri Highway Patrol said. And a business owner said looters tried to make off with his cash register. It was the third time the business had been broken into in the last six weeks, Johnson said.

The St. Louis suburb has been in the national spotlight since August, when Officer Darren Wilson shot 18-year-old Michael Brown in the street.

Protests carried on for weeks and, in the days just after the shooting, devolved into looting, vandalism and arrests. The tenor of the demonstrations eventually calmed, but on Tuesday, tensions flared again.

Protesters said they were angry that one of three makeshift memorials at the site where Brown was shot burned earlier in the day.

Across town, about 2 miles from the normal protest route, a museum and custard shop called the Whistle Stop was set on fire, and "gasoline appears to have been poured around that facility," Johnson said.

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Before it burned Tuesday, the Michael Brown memorial consisted largely of stuffed animals and flowers.

Officers were able to douse the small fire with a garden hose, the trooper said. The St. Louis County Bomb and Arson Squad was called in to investigate, CNN affiliate KSDK reported.

Last month, the Whistle Stop's owner, Charles Stark, 26, spoke to CNN and described Ferguson as a place that bands together in tough times, "neighbor helping neighbor."

Back on the protest route, police faced threats to their safety as violent people amid the protest made threatening remarks "about shooting police, about burning down the town," Johnson said.

When some of the people making threats got too close to officers, "the decision was made to disperse the crowd," he said.

One person was arrested for assault, inciting a riot and failure to disperse, Johnson said, adding that he couldn't remember why the other four were arrested, but he believed it was for failure to disperse.

The cause of the fire at the memorial remains under investigation. In a statement, Police Chief Thomas Jackson said one of his sergeants responded to the fire and attempted to put it out with a portable fire extinguisher before alerting the Fire Department.

"We're all saddened by the fire that was reported at one of the Michael Brown memorials this morning and are trying to obtain as much information as possible to determine what happened," the statement said.

Some protesters, however, already had ideas about what happened.

"We know it wasn't an accident. You know how many people live over there that seen it from the beginning? I mean it's just a big ol' flame. You could tell the way it was set," a protester told KSDK.

Johnson bristled at the suggestion that police were responsible for or ambivalent about the fire, saying numerous residents "stood there for 11 minutes watching it burn." Some residents went door-to-door telling people that police were behind the blaze when they "could've put it out with a cup of water," he said.

It was in that area, along Canfield Drive, that police in bulletproof vests saw muzzle flashes as people fired at them.

"I was there. I took cover," Johnson said.

Officers will continue to strive to protect public safety as well as the freedoms of speech and expression, Johnson said, but if certain violent elements among the protesters -- and he stressed that most of the demonstrators were, indeed, peaceful -- authorities will have no choice but to respond.

"If that means officers will respond in riot gear, they will," he said. "We cannot have nights like last night. We can't have actions like last night that can result in injury or death. Those will not be tolerated."