Although he was relieved the killing of the armed suspect in Dallas assured that no other police officers or bystanders would be harmed, Cohen says he's anxious about what might happen next.
Transit Police Officer Brent Thompson, 43, worked for the Dallas Area Rapid Transit (DART) system since 2009 and was the first officer killed in the line of duty since the transit system formed a police department in 1989, DART said on its website.
He told the crowd of more than 100 people that voting in November "as though our lives depend upon it" would be the best way to reform policing practices.
The protesters are now in a stand-off with police as they block a major road.
Law enforcement sources believe Johnson had been planning the attack on police for some time but that it was the police shootings of black men in Baton Rouge, Louisiana, and Minnesota that pushed him over the edge, CBS News homeland security correspondent Jeff Pegues reports.
But later, all attention focused on Johnson, and state and federal officials said the entire attack appeared to be the work of a single gunman.
At a morning press conference, Dallas Police Chief David Brown said the shooter was killed by a police "bomb robot", but had told the police hostage negotiator that he "was upset about Black Lives Matter" and that he "wanted to kill white people, especially white officers". According to Brown, Johnson claimed he was "not affiliated with any groups", adding that he "did this alone".
The authorities initially blamed multiple "snipers" for Thursday's attack, and at one point said three suspects were in custody.
Investigators later sealed off and searched his home, carrying away several bags of material.
The Rev. Jeff Hood, an organizer of Thursday night's protest, said he had been chatting with some of the police officers on the street when gunfire erupted. "One of the things it can do is put our police officers in harm's way, and we have to be very careful about doing that". Support can be drummed up for institutions of the capitalist state when tragedies like the murders of random law enforcement officers take place. Two civilians were also wounded.
He later ordered all flags on public buildings to be flown at half-mast.
President Barack Obama will cut short his European trip and visit Dallas early next week.
Reacting to the latest shootings, presumptive Democratic presidential nominee Hillary Clinton said: "There is too much violence, too much hate, too much senseless killing, too many people dead who shouldn't be".
McMillon described Sterling as a good father to their son Cameron, calling them the "Doublemint twins" because they liked to eat snacks together. But, Obama spoke out again less than 12 hours later when shootings in Dallas rocked the nation.
Alton Sterling, Philando Castile, Trayvon Martin, Mike Brown, Eric Garner, Freddie Gray, Tamir Rice, Sandra Bland, Sean Bell ... and unfortunately so many more. "This is an American issue that we should all care about".
In Minnesota, Philando Castile's girlfriend streamed video to Facebook after he was shot by a police officer Wednesday.
In Baton Rouge, where Alton Sterling was fatally shot Tuesday, protesters marched near the police headquarters.
Both incidents were captured on video, reigniting what has become a national debate.