Trybus was later charged with assault and disorderly conduct. Officer Connor was caught on camera last month, seemingly standing by as another man scolded Mia Irizarry for wearing a Puerto Rico flag T-shirt.
When the woman complains to a Cook County Forest Preserves officer that the man is harassing her and questioning her citizenship, the officer doesn't respond.
In the unedited video, Mr Connor is seen trying to reassure the victim that Mr Trybus does not pose a threat to her safety.
"You should not be wearing that in the United States of America", he said.
"The 10-year veteran officer resigned late Wednesday, according to Cook County Forest Preserve District officials, rather than fight to keep his badge over the encounter that has been recorded and viewed millions of times, drawing worldwide scorn for Connor's inaction", the outlet reported.
'Officer, I feel highly uncomfortable, can you please grab him, please?' she asks the cop again.
Officer Patrick Connor was supposed to have a sit down with his bosses about the incident, but instead he quit on July 11, The Chicago-Sun Times reported.
The woman, who apparently is the one recording the encounter, can be heard talking but her face is not shown.
"I felt so drawn out", she said in the interview.
Irizarry asks a park police officer to help, saying "I am renting this area and he's harassing me about the shirt that I'm wearing".More news: Roger Federer loses five-set match to Kevin Anderson
More officers arrived and arrested the man, who was described as drunk.
Coming closer, the man says: "You're not gonna change us, you know that?"
Almost half of Americans are unaware that Puerto Ricans are United States citizens, according to polling taken after the island was rocked by a hurricane last September.
The female officer then interviews Irizzary, who tells her that the first officer on the scene stood "watching the whole thing happen".
It's not clear when the investigation into the officer's response will be completed.
"We don't know what was happening outside the video; we don't know what was going on inside his [Conner's] head at the time", Cummings told CNN.
"We might start with the fine example presented by Mia Irizarry and her family, who handled this ugly incident with great composure, never stooping to the level of the man who attacked her", he said in a written statement.
There was also a comment at the end "Why don't you and your people go back where you came from".
Cook County Commissioner Jesús "Chuy" García had called for such charges against Trybus a day earlier.
The appalling incident of June 14 and the inaction of one former member of the Forest Preserves Police should not diminish the greatness of the Preserves, nor dissuade people from enjoying our 70,000 acres of nature and beauty. She said the group politely complied but her Puerto Rico shirt appeared to act as a trigger to the man. The island is a US commonwealth, and Puerto Ricans are American citizens.