Buffalo police commissioner Joseph Gramaglia told ABC News on Monday that Mr Gendron, 18, could have continued shooting in the area around the Tops supermarket on Saturday, where 10 people were shot dead.
“We have uncovered information that if he escaped the [Tops] supermarket, he had plans to continue his attack,” Mr Gramaglia said. “He had plans to continue driving down Jefferson Ave to shoot more Black people ... possibly go to another store [or] location.”
Mr Gendron, who was taken into custody on Saturday dressed in military fatigues, allegedly shot four people in the parking area before continuing inside the store.
All of the deceased were Black, authorities said, and Mr Gendron was believed to have travelled 200 miles (320 km) from his home outside Binghamton to the store. The area is a predominantly Black neighbourhood.
Police told CNN that he had expressed hatred towards Black people after being taken into custody, and that evidence reviewed as part of the investigation into the shooting suggested Mr Gendron was “studying” previous shootings and attacks.
A 180-page manifesto reportedly written by Mr Gendron and shared online also outlined the “Great Replacement Theory” shared by racist conspiracy theorists. Video meanwhile showed his gun with the number “14”, a known white supremacist symbol.
He was pleaded not guilty to first-degree murder charges. If convicted, he could face life imprisonment.
The investigation is ongoing.