"This was not an act of terrorism... We've ruled that out," Deputy Police Chief Brett Zimmerman told reporters.
But "we have determined that this is an intentional act."
The Las Vegas Strip, lined with casinos and fancy hotels, attracts throngs of tourists from around the world and is usually abuzz with activity.
The incident occurred yesterday, the same night that Las Vegas hosted the popular Miss Universe pageant, which saw the crowning of Miss Philippines.
"You think it's a show, because you're in Vegas," Qureshi was quoted as saying. "But then I saw some people fly in the air."
The suspect was in custody, speaking with detectives and undergoing several tests, including for alcohol.
Police - urging more witnesses to come forward - described her as an African American woman in her 20s who was driving a 1996 Oldsmobile with Oregon license plates.
The toddler in the car was unharmed, they said.
He said the woman had yet to be charged.
Zimmerman told reporters that one person had been killed and that 26 people were taken to area hospitals after the incident.
He said six of those injured and listed in critical condition were now stable.
Police had earlier put the number of people hurt at 37.
Local media outlet 8 News NOW reported that the incident occurred shortly after 6:30 pm between Planet Hollywood, where the Miss Universe pageant took place, and the Paris Las Vegas Hotel.
The suspect initially left the scene, police confirmed.
Another local media outlet, KTNV, reported that the University Medical Center, one of three hospitals where the injured were taken, said most of its patients were from Montreal in Canada.
Cars were diverted near the Paris Las Vegas hotel, where the Strip was shut down as first responders rushed to the chaotic scene, according to CNN.
Images from the scene showed a slew of ambulances and medical personnel attending to the injured amid the backdrop of the city's world famous hotels.
