FERGUSON, Mo--Violence erupted here once more overnight, even as Missouri National Guard troops arrived, the latest in a series of quickly shifting attempts to quell the chaos that has upended this St. Louis suburb for more than a week.

In the days since an unarmed black teenager, Michael Brown, was shot to death by a white police officer here on Aug. 9, an array of state and local law enforcement authorities have swerved from one approach to another: taking to the streets in military-style vehicles and riot gear; then turning over power to a Missouri State Highway Patrol official who permitted the protests and marched along; then calling for a curfew.

Early Monday, after a new spate of unrest, Gov. Jay Nixon said he was bringing in the National Guard. Hours later, he said that he was lifting the curfew and that the Guard would have only a limited role, protecting the police command post.

Although the tactics changed, the nighttime scene did not.

Late Monday night, peaceful protests devolved into sporadic violence, including gunshots, by what the authorities said was a small number of people, and demonstrators were met with tear gas and orders to leave. Two men were shot in the crowd, officials said in an early-morning news conference, and 31 people, some from New York and California, were arrested. Fires were reported in two places. The police were shot at, the authorities said, but did not fire their weapons.

“We can’t have this,” said Capt. Ronald S. Johnson, the Highway Patrol official, who stood near a table that held two guns and a Molotov cocktail that had been seized. “We do not want to lose another life.”

Captain Johnson, who is coordinating security operations, gave no sense of whether the police would change their tactics again on Tuesday. But he urged peaceful protesters to demonstrate during daylight hours so as not to give cover to “violent agitators,” and he pledged, despite the repeated nights of tumult, “We’re going to make this neighborhood whole.”

Adding to the turbulence was confusion over the curfew. Although it was no longer in force, the police demanded around midnight that the crowd disperse, a move the authorities attributed to increasingly unsafe conditions.

Also on Monday, more details emerged from autopsies performed on Mr. Brown’s body. One showed that he had been shot at least six times; another found evidence of marijuana in his system.

In Washington, President Obama said that Attorney General Eric H. Holder Jr. would go to Ferguson on Wednesday to meet with F.B.I. agents conducting a federal civil rights investigation into the shooting. He seemed less than enthusiastic about the governor’s decision to call in the National Guard.

Mr. Obama said he had told Mr. Nixon in a phone call on Monday that the Guard should be “used in a limited and appropriate way.”

He added that he would be closely monitoring the deployment.

“I’ll be watching over the next several days to assess whether in fact it’s helping rather than hindering progress in Ferguson,” said Mr. Obama, who emphasized that Missouri, not the White House, had called in the Guard.

He again tried to strike a balance between the right to protest and approaches to security.

“While I understand the passions and the anger that arise over the death of Michael Brown, giving in to that anger by looting or carrying guns and even attacking the police only serves to raise tensions,” Mr. Obama said.

As darkness set in along West Florissant Avenue, one of the city’s main thoroughfares and a center of the weeklong protests, demonstrators were required to keep moving.

After more than an hour of peaceful protests, some in the crowd began to throw bottles at the police, who brought out armored vehicles and tactical units. But many peacekeepers in the crowd formed a human chain and got the agitators to back down.

At another point, as protesters gathered near a convenience store, some of them threw objects; the police responded with tear gas.

And near midnight, the police began announcing over loudspeakers that people needed to leave the area or risk arrest after what the police said were repeated gunshots and a deteriorating situation.

A few blocks away, at the police command post, National Guard members in Army fatigues, some with military police patches on their uniforms, stood ready but never entered the area where protesters were marching. State and local law enforcement authorities oversaw operations there.

Residents seemed puzzled and frustrated by the continually changing approaches, suggesting that the moving set of rules only worsened longstanding tensions over policing and race in this town of 21,000.

Explaining his decision to call in the National Guard, Mr. Nixon recounted details of the unrest on Sunday night and described the events as “very difficult and dangerous as a result of a violent criminal element intent upon terrorizing the community.”

Yet Mr. Nixon also emphasized that the Guard’s role would be limited to providing protection for the police command center, which the authorities say was attacked. Gregory Mason, a brigadier general of the Guard, described the arriving troops as “well trained and well seasoned.”

“With these additional resources in place,” said Mr. Nixon, a Democrat in his second term, “the Missouri State Highway Patrol and local law enforcement will continue to respond appropriately to incidents of lawlessness and violence and protect the civil rights of all peaceful citizens to make their voices heard.”

While Mr. Obama and other leaders called for healing and more than 40 F.B.I. agents fanned out around the city to interview residents about the shooting, emotions remained raw, and the divide over all that had happened seemed only to be growing amid multiple investigations and competing demonstrations.

A recent survey by the Pew Research Center for the People and the Press showed that Americans were deeply divided along racial lines in their reaction to Mr. Brown’s killing. It showed that 80 percent of blacks thought the case raised “important issues about race that need to be discussed,” while only 37 percent of whites thought it did.

Blacks surveyed were also less confident in the investigations into the shooting, with 76 percent reporting little to no confidence, compared with 33 percent of whites. (Source: The New York Times)

Story Date: August 20, 2014