"Racial divisions have gotten worse, not better", Trump said.
Asked by CNN anchor Wolf Blitzer if Comey's assessment that she careless was accurate, Clinton did not answer directly.
Syrian state media says 30 people have been killed in rocket fire overnight in Aleppo, the country's largest city and the scene of some of the fiercest fighting in Syria's civil war, now in its sixth year.
The confrontation followed the deadliest day for U.S. law enforcement since the 9/11 2001, terrorist attacks.
"Those police officers were protecting a peaceful protest" and that is "a hallmark of America", she said during the CNN interview. Both Northam and Stewart have said they plan to run for governor in Virginia in 2017.
However, she also called for Americans to work to end "systemic racism" and rebuild communities.
President Barack Obama, speaking from Warsaw, Poland, called the killings a "vicious, calculated, despicable attack on law enforcement". "The senseless, tragic deaths of two motorists in Louisiana and Minnesota reminds us how much more needs to be done".
On Tuesday, Alton Sterling was shot in Louisiana after being pinned to the pavement by two white officers, an altercation that was captured on cellphone video. The following day, Castile was fatally shot in a vehicle by a Minnesota officer, with the aftermath livestreamed on Facebook by his girlfriend.
While Dallas police officers were responding to a shootout that killed five of their own, a Donald Trump surrogate in Virginia wasted no time in politicizing the event for his candidate's benefit. Two civilians were also hurt. He was supposed to rally supporters in Doral as a primary candidate in March, but called off that event to spend more time campaigning in Ohio.
Josh Greenfield, who drove with his friends from Syracuse, N.Y., to try to get autographs, wondered why Clinton couldn't have used the event to address the Dallas tragedy.
Democrat Hillary Clinton and Republican Donald Trump each offered a response to the sniper attack during a Black Lives Matter protest in downtown Dallas Thursday night. We need jobs, and we are going to produce those jobs. Marco Rubio of Florida said of this week's shootings.
But Clinton also warned that there was a "terrible disconnect" between police officers and the people they are meant to protect. Too many Americans feel like they've lost hope.
President Barack Obama talks with France's President Francois Hollande, right, as they participates in a North Atlantic Treaty Organisation family photo at the Presidential Palace in Warsaw, Poland, Friday, July 8, 2016. He said the focus Friday should be on the victims and their families, but added that "when people are armed with powerful weapons, unfortunately, it makes attacks like these more deadly and more tragic". "We must restore the confidence of our people to be safe and secure in their homes on the street", he said.