Four police officers killed by snipers in Dallas

Four police officers were fatally shot and seven wounded by snipers in Dallas, authorities said, during one of many demonstrations held in cities across the United States against the second fatal police shooting of a black man in two days.

Dallas Police Chief David Brown said in a statement two snipers in elevated positions shot 11 officers, killing three. Police said on Twitter later a fourth officer had died. At least one more was in surgery.

Brown also said later one suspect was cornered in a garage in downtown Dallas, although there was no word on the second reported sniper.

Television footage showed a heavy police presence, with officers taking cover behind vehicles on the street. "Our thoughts and prayers are with the Dallas law enforcement community and the Dallas Area Rapid Transit officers killed and injured this evening,” Governor Greg Abbott said in a statement.

The shooting happened as largely peaceful protests unfolded around the United States after the shooting of Philando Castile, 32, by police near St. Paul, Minnesota, late on Wednesday. His girlfriend posted live video on the internet of the bloody scene minutes afterward, which was widely viewed.

Castile’s death occurred within a day of the shooting of Alton Sterling, 37, in Baton Rouge, Louisiana. Sterling was killed during an altercation with two white police officers. Graphic video of that incident caused an outcry on social media. In Chicago, protesters shut down a stretch of the Dan Ryan Expressway - one of Chicago's main arteries - for about 10 minutes on Thursday.

In New York, several hundred protesters blocked traffic in Times Square in the heart of Manhattan, chanting "Hands up, don't shoot." More than a dozen arrests were made, the New York Police Department said.

In St. Paul, about a thousand people gathered outside the governor's mansion, chanting "Hey hey, ho ho, those killer cops have got to go," and other slogans.

style="margin-bottom: 0.0001pt; text-align: justify;">Minnesota Governor Mark Dayton made a brief appearance in an attempt to quell the crowd. He said earlier a state investigation was already under way. "Would this have happened if the driver and the passengers were white? I don’t think it would have," Dayton told reporters, speaking of the Castile shooting.

"So I’m forced to confront that this kind of racism exists, and it's incumbent upon all of us to vow and ensure that it doesn't happen and doesn't continue to happen," he said. State investigators later identified Minneapolis area police officer Jeronimo Yanez as the patrolman who fatally shot Castile during a traffic stop.

U.S. President Barack Obama described the killings as tragedies. "All of us as Americans should be troubled by these shootings, because these are not isolated incidents. They're symptomatic of a broader set of racial disparities that exist in our criminal justice system," he said after arriving in Poland for a NATO summit.

The use of force by police against African-Americans in cities from Ferguson, Missouri, to Baltimore and New York has sparked periodic and sometimes violent protests in the past two years and has spawned the Black Lives Matter movement.

Anger has intensified when the officers involved in such incidents have been acquitted in trials or not charged at all. "I was already fuming when I woke up this morning over Baton Rouge, but for it to happen here again just pushed me right over the edge," said truck driver Thomas Michaels, 42, who was among the protesters in St. Paul.

"We live in a racist society where black lives don't matter, my kids lives don't matter and I'm sick of it. I don't even know if it can be fixed," he said. Another protester, retail worker Tanya McDonald, 28, said: "What gets me is how many people are failing to see that this is happening almost every day. We're dying, we're being killed off by people hiding behind a badge and no one's doing anything to stop it."

The Washington Post said Castile was at least the 506th person and 123rd black American shot and killed by police so far in 2016, according to a database it has set up to track such deaths.