Many questions hang in the air following Tuesday night's explosive protests outside the Trump rally, including, how did a peaceful protest take such a turn?

A total of four people were arrested: Daireus Stokes, 18, and Pamela Robertson, 25, are facing aggravated assault charges. Derrick Pacheco, 34, was arrested for disorderly conduct, and a teenager was also arrested for criminal damage.

Police Chief Jeri Williams said it all started when someone in the crowd threw tear gas at police officers. It is unclear who the person was, and where he or she got the tear gas.

Police responded with pepper spray and pepper balls, which were confused by some for rubber bullets.

Many are praising the police department's response, but some question whether officers used too much force.

The ACLU is calling for an investigation into reports of excessive force.

Isabella Maldonado, a retired police captain with 22 years of law experience says no.

"The difference between a peaceful protest and a disorderly protest can be one person," said Maldonado.

She says all it takes is one person to throw something at police or incite others in the crowd to make the difference between a peaceful protest and a unlawful assembly. Once that happens, it doesn't matter if you are a peaceful protester -- you are considered part of the unlawful assembly, and police will disperse the crowd.

"A lot of people didn't understand [and say ] 'Well I was peacefully demonstrating, and I was exercising my constitutional rights,'" she said. "Well the answer is, it's because you were part of something that then was declared to be unlawful."

Maldonado said once an unlawful assembly is called, police will then generally give people a warning that tear gas or pepper spray will be used, but legally, they are not required to give that warning.

"This is the same as any police officer using deadly force, even though this isn't deadly force, using it as an analogy, before an officer would shoot, ideally, he would say 'Police, drop your weapon,'" she said. "But sometimes there may not be time to give those sorts of orders. That's why there's no requirement for that in an urgent situation."

Phoenix Police Chief Jeri Williams said she welcomes a review of the incident.

"I would say my employees and the officers out there last night are A+," she said. "They did exactly what I asked them to do and that was be professional and make sure they took care of the scenario and situation. I welcome the review of our actions and activities, which we're willing to share with people, and I'm also willing to meet with individuals who have a different opinion."