Payton Gendron, the 18-year-old who allegedly gunned down 10 people — all of whom were Black — at a Tops grocery store in Buffalo, New York, would have continued his rampage had he not been stopped, Buffalo Commissioner Joseph Gramaglia told ABC News.

“We have uncovered information that if he escaped the [Tops] supermarket, he had plans to continue his attack,” Gramaglia said. “He had plans to continue driving down Jefferson Ave. to shoot more Black people … possibly go to another store [or] location.”

Authorities are calling Saturday’s massacre a “racially motivated hate crime.”

“This was well planned … by a sick person,” Gramaglia said.

Officers responded to the scene within one minute and when they approached the suspect, the teen put his assault rifle to his neck, according to the commissioner.

The commissioner praised the responding officers who he said deescalated the situation and convinced the gunman to drop his weapon, saving countless lives.

Multiple high-capacity magazines were recovered on Gendron and in his car, the commissioner said. While he declined to say what evidence pointed to additional shooting plans, the commissioner said investigators have been going through his phone and other electronics.

The teen is from Conklin, New York, which is 200 miles east of Buffalo.

Police determined Gendron arrived in Buffalo on Friday via license plate reader and other evidence, the commissioner said. Police are still working to determine where he stayed overnight before Saturday’s attack.

Shonnell Harris Teague, an operations manager at Tops, said she saw Gendron sitting on a bench outside of the store on Friday afternoon. She said he was there for several hours with a camper bag on his back, dressed in the same camouflage outfit he wore Saturday.

She said Gendron entered the store Friday evening, and appeared as if he was bothering customers. Teague asked him to leave and he did so without an argument.

The next time Teague saw him was on Saturday as a mass shooting unfolded at her store. She escaped out of the back when she saw Gendron.

“I see him with his gear on and his gun and how it was all strapped on. … I seen all the other bodies on the ground. … It was just a nightmare,” she said.

Gendron has been arraigned on one count of first-degree murder and is due back in court on May 19.

ABC News’ Miles Cohen contributed to this report.