Police fire tear gas in US town as Obama calls for calm

Police fired tear gas Monday in another night of unrest in a Missouri town where a white police officer shot and killed an unarmed black teenager, just hours after President Barack Obama called for calm.
      
US National Guard troops rolled into Ferguson earlier in the day, but they kept a low profile as police in riot gear dispersed about 100 demonstrators around 11 p.m. (0400 GMT), arresting several of them.
      
Several rounds of gunfire -- described by a senior police officer as "potshots in the area" -- were heard, adding to tension that has gripped this St Louis suburb since the fatal shooting of 18-year-old student Michael Brown.
      
His death, and the heavy-handed police response, has reignited a national debate about race and law enforcement in the United States.
      
Obama, the nation's first African-American president, said he was sending Attorney General Eric Holder to Ferguson on Wednesday as Washington pursues a civil rights investigation into the case.
      
Obama said there was no excuse for local police to employ "excessive force" and urged the state of Missouri to make only "limited" use of the National Guard.
      
By late afternoon, about 200 Missouri National Guard soldiers arrived in Ferguson, with snipers seen posted on rooftops near a police command center. The troops are operating under Missouri Highway Patrol supervision.
      
The reinforcements allowed State Governor Jay Nixon to lift an overnight curfew, but tempers were still running high amid controversy over Brown's death.
      
"They're supposed to protect the American citizens, but they're fighting a war with unarmed citizens," said Ron Henry, who wore a T-shirt...