A person of interest in currently being questioned, police said.
The three were shot and wounded in an attack near the 22nd Chicago Police District headquarters at about 5:50 a.m., Brown said during a press conference later Wednesday.
The car was driving on a ramp near northbound I-57 and 119th Street, police said. The federal agents and CPD officer were in an unmarked car following a gun suspect when the shooting took place, according to several law enforcement sources. The shooter is said to have opened fire on the tail car-wounding all three plainclothes investigators who were inside including the two senior ATF agents.
While there is no public description tonight of why the suspect started shooting, several law enforcement sources told the I-Team he has told detectives he was unaware he was shooting at police and the officers did not return fire. Sources said that this shooting transpired during a current investigation by the Chicago Crime Gun Strike Force. The Strike Force was set up in 2017, staffed by dozens of federal agents and specially-deployed Chicago police officers who aim to intercept illicit guns from flooding Chicago streets and arming street gangs and drug organizations.
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In 2018, one of the unit's federal agents was wounded in the Back of the Yards neighborhood during an investigation. ATF's strike force operations here are expected to get added support under President Biden's new national anti-gun violence plan.
RELATED: ATF agent shot in Chicago was on special strike force to battle guns
"Officers were fired upon from the street towards the on ramp while they were in their car on-ramping onto the freeway I-57," Brown said.
One ATF agent was hit in the hand and another was struck in the torso. One agent is a man and one is a woman, police said.
The CPD officer was grazed on his head, Brown said.
While wounded, they were able to get to the Morgan Park Police Station about a mile away. The three were taken to Christ Hospital after the shooting, and their injuries are not life threatening. They have since been released from the hospital, federal officials told the I-Team Wednesday afternoon.
Law enforcement sources said a suspect vehicle was found parked on the street at 207 E. 89th Place, but Brown would not confirm that detail.
A person of interest was in police custody by mid-afternoon, but investigators continued canvassing the South Side block for hours, looking for any witnesses or surveillance video. Just after 5:30 p.m., agents entered a home, emerging with brown envelopes used to collect evidence.
WATCH: CPD supt. gives update after officer, ATF agents hurt in shooting
An ATF spokeswoman said the two injured are "senior" agents. The officer has also been with the CPD for quite some time.
Officials declined to provide information about the investigation in which the three were involved, saying only that they were working undercover.
At least three dozen CPD officer shot at in 2021 so far
Brown said this is the 36th CPD officer shot at this year; nine have been hit.
"The officers are performing their jobs at the highest level; they are risking everything to protect the people of Chicago," he said. "No one would do what these officers do right now. This is a very challenging time to be in law enforcement, but they are rising to the challenge."
The danger for police is ever-present. The Chicago Police Memorial Foundation reported last year that 79 officers were shot, 10 were hit by gunfire.
"The fear is gone," said Sgt. James Calvino, president of the Chicago Police Sergeants' Association. "The fear of being arrested, they don't care any longer. It's like a turn style at a loop office building. You go in, spin around, and you're right back out."
Police staffing is another concern. Calvino said the city's 2021 budget calls for 1,320 sergeants. He says as of July 1st, there were 1, 219 - about 100 short, adding that more sergeants are retiring soon, and about 30 will be promoted to lieutenants
Alderman Matt O'Shea, with the 19th Ward, said he was glad those injured will be OK, but called on President Joe Biden for help with Chicago violence.
He said community members are afraid to let their kids play outside.
"Another example of just where we're at as a city," he said. "Our communities are under siege. Our police officers are under siege; they're outmanned and they're outgunned. We have 100,000 gang members in the city of Chicago right now; they're all armed. They're emboldened. They have absolutely no fear, no respect for life, no respect for the communities that they come from."
WATCH: Alderman says communities, police 'under siege' after police shooting
In a statement, the mayor's office said Wednesday night: "During today's meeting, Mayor Lightfoot gave President Biden an update on the ATF agents' medical conditions and thanked him for his additional supports in the city's fight against gun violence. She lauded the President for listening to the needs of cities and for the federal government's recent announcements on public safety reform."
South Side neighborhood on edge
Those who live near 89th and Indiana Avenue are worried about the crime, as the alleged suspect car was parked nearby.
Several Chicago Police Department squads, in addition to a SWAT team, converged on the South Side neighborhood and began searching alleys, backyards and at least one house for the suspect or suspects.
"Just concerning, you know - I grew up around here, so I've seen the changes," neighborhood resident Zach Henry said. "But I remember how it used to be - quiet, senior working class neighborhood. It's pretty different with all of this going on."
Police remained on the scene about 11 a.m. Wednesday and said they did interview several residents whose homes are located near where the car was found.
Community activist and St. Sabina Pastor Father Michel Pfleger said "Illinois is in a state of emergency and the Windy City is becoming known as a bloody city."
He went on to say, "Governor Cuomo declared a gun violence state of emergency in New York State and is committing $139 million to reverse the trend of rising shootings and murders across the state. 886 people have been shot this year so far in New York, through July 4th. In Chicago, 1743 have been shot so far this year and 357 killed. Governor Pritzker needs to declare gun violence an emergency in Illinois and commit funds to reverse the trend here as well as call for an emergency summit of state, county, and city officials and community leaders and residents to come up with a comprehensive plan and strategy."