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."

       Usman Khan/AFP via Getty Images

       Mourners gather near a Tops supermarket in Buffalo, New York, May 15, 2022. Grieving residents held vigils Sunday after a white gunman shot dead 10 people at a grocery store in a racially-motivated rampage.

       Authorities are calling Saturday's massacre a "racially motivated hate crime."

       MORE: Mother, daughter escape gunfire at Buffalo store: 'By the grace of God we got out'

       "This was well planned ... by a sick person," Gramaglia said.

       Matt Rourke/AP

       A person pays respects outside the scene of a May 14 mass shooting at a supermarket in Buffalo, N.Y., May 15, 2022.

       Scott Olson/Getty Images

       People pray across the street from Tops supermarket on May 15, 2022 in Buffalo, New York, the day after 10 people were killed and three wounded when a gunman opened fire.

       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.

       Robert Bumsted/AP

       FBI Investigators enter the Tops supermarket in Buffalo, N.Y. on Monday, May 16 2022.

       MORE: Hate, online radicalization: Buffalo shooting reflects law enforcement's fears

       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.

       MORE: Why a retired police officer who confronted Buffalo gunman is being hailed a hero

       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.

       Matt Rourke/AP

       People embrace outside the scene of a mass shooting at a supermarket a day earlier, in Buffalo, N.Y., May 15, 2022.

       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.

       MORE: Shoppers who were regulars, a retired police officer: What we know about the victims of the Buffalo shooting, an alleged hate crime

       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.

       Scott Olson/Getty Images

       Police and FBI agents continue their investigation of the shootings at Tops market that killed 10 people on May 15, 2022 in Buffalo, New York.

       "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.