The FBI is at the scene and assisting the investigation, she said.
Montalvo did not reveal the suspect's identity, but told reporters, "he is a lawyer and there were issues concerning his law firm."
Police said nine people were injured in the shooting, one critically and one seriously.
There are no reports of additional suspects, police said.
Witness: Gunshots flew by my face
It was still dark when Antoine Wilson drove through the neighborhood Monday morning and suddenly found himself in the middle of a gunfight.
"I didn't realize I was right there by the shooter," he told CNN affiliate KTRK. "Gunshots, I literally hear the gunshots pass my face, 'cause I'm leaning out the window looking, trying to see."
Then he saw police running down the street.
"You (could) hear people screaming," he told CNN affiliate KTRK. "I'm talking about steady gunshots, just steady shooting, back and forth ... police and the shooter. Helicopters were everywhere."
Firefighters and police swarmed the scene as gunfire rang out, KTRK reported.
"I'm still shaken up," Wilson told KTRK. "I've never been in no standoff like this before."
Authorities to residents: Shelter in place
Early reports indicated the shooting took place in a strip mall parking lot near the corner of Weslayan Street and Bissonnet Street in southwest Houston.
Montalvo later said it's possible the shooting had occurred on a nearby street.
"As the victims left, the ones that could, possibly, came to this location," she told reporters at a press briefing near the strip mall.
An emergency alert from the city warned people to avoid the area. Officials advised residents to stay indoors or seek shelter while authorities investigate.