It was the deadliest day for police in the United States since the Sept. 11, 2001, attacks on New York and Washington.

A video taken by a witness shows a man with a rifle crouching at ground level and charging at and then shooting another person who appeared to be wearing a uniform. That person then collapsed to the ground.

Reuters could not immediately confirm the authenticity of the video.

A total of 12 police officers and two civilians were shot during the attack, Dallas Mayor Mike Rawlings said. Three of the officers who were shot were women, he said.

One of the dead officers was identified as Brent Thompson, 43. He was the first officer killed in the line of duty since Dallas Area Rapid Transit formed a police department in 1989, DART said on its website. Thompson joined DART in 2009.

Rawlings told CBS News the people in custody, including one woman, were “not being cooperative” with police investigators. He said the assailant who was dead was being fingerprinted and his identity checked with federal authorities.

There was no sign of international links to the attacks, U.S. officials said on Friday.

Experts on extremist groups said such attacks are not necessarily carried out by an organization and are often the work of individuals. Black groups have not been linked to any recent violent attacks in the United States, they said.

‘DESPICABLE ATTACK’

President Barack Obama, who was traveling in Poland, expressed his “deepest condolences” to Rawlings on behalf of the American people.

“I believe I speak for every single American when I say that we are horrified over these events and we are united with the people and police department in Dallas,” he said.

Obama said the FBI was in contact with Dallas police and that the federal government would provide assistance.

“We still don’t know all of the facts. What we do know is that there has been a vicious, calculated and despicable attack on law enforcement,” he said.

A large swath of downtown Dallas was closed to traffic and pedestrians on Friday as police gathered evidence.

The shooting happened as otherwise largely peaceful protests unfolded around the United States after the police shooting of Philando Castile, a 32-year-old black man, on Wednesday during a traffic stop near St. Paul, Minnesota.

The day earlier, police in Baton Rouge, Louisiana, shot dead Sterling, 37, while responding to a call alleging he had threatened someone with a gun.

Over the last two years, there have been periodic and sometimes violent protests over the use of police force against African-Americans in cities from Ferguson, Missouri, to Baltimore and New York. Anger has intensified when the officers were acquitted in trials or not charged at all.

Dallas is a pioneer in training its police officers in de-escalation techniques, Rawlings told reporters, saying the department had the lowest number of police-involved shootings of any large American city.

Presidential candidates Republican Donald Trump and Democrat Hillary Clinton canceled planned events following the attack.

(Reporting by Brendan O’Brien in Milwaukee, Eric M. Johnson in Seattle, Jon Herskovitz in Austin, Texas, Letitia Stein in Tampa, Florida and Laila Kearney in New York; Writing by Scott Malone and Jon Herskovitz; Editing by Alison Williams and Jeffrey Benkoe)