Payton Gendron, 18, already faces mandatory life term if convicted on state charges over deaths of 10 Black people in May attack

The white gunman who killed 10 Black people in a racist attack at a Buffalo supermarket in May has been charged with federal hate crimes and could face the death penalty, according to a criminal complaint filed on Wednesday.

Payton Gendron already faced a mandatory life sentence without parole if convicted on state charges in the 14 May shooting which also wounded three survivors – one Black, two white.

The US attorney general, Merrick Garland, was in Buffalo on Wednesday to visit families of the 10 people killed. He was expected to address the federal charges during the visit.

Gendron’s radical, racist worldview and extensive preparation for the attack at the Tops Friendly Market are laid out in documents he apparently posted online.

The documents embrace a conspiracy theory about a plot to diminish white Americans’ power and “replace” them with people of color, through immigration and other means.

The posts detail months of reconnaissance, demographic research and shooting practice for a bloodbath meant to scare anyone not white and Christian into leaving the country.

Gendron drove more than 200 miles from his home in a nearly all-white town near the New York-Pennsylvania border to a predominantly Black part of Buffalo. There, authorities say, he killed shoppers and workers using an AR-15-style rifle, wearing body armor and livestreaming the carnage from a helmet-mounted camera.

The 18-year-old surrendered to police as he exited the supermarket.

He has pleaded not guilty to a state domestic terrorism charge, including hate-motivated domestic terrorism and murder.

According to the online documents attributed to Gendron, he scouted the supermarket in March, drawing maps and even counting the number of Black people he saw.

According to an affidavit filed with the federal criminal complaint, FBI agents executing a search warrant at Gendron’s home the day after the shooting found a note in which he apologized to his family and stated that he “had to commit this attack” because he cares “for the future of the white race”.

Gendron signed the note and addressed it to his family, the affidavit said.

Agents at the home in Conklin, New York, also found a receipt for a candy bar purchased from the Buffalo supermarket on 8 March, the day Gendron said in an online diary he went to scout out the store, as well as hand-drawn sketches of the store’s layout, the affidavit said.

Ten days after the attack, another 18-year-old with a semi-automatic rifle opened fire at an elementary school in Uvalde, Texas, killing 19 children and two teachers.

Soon after, the governor of New York, Kathy Hochul, signed 10 public safety bills including one prohibiting New Yorkers under age 21 from buying semi-automatic rifles and another that revised the state’s “red flag” law, which allows courts to temporarily take guns from people who might be a threat to themselves or others.

The US Senate followed on 12 June with a framework agreement on more modest federal gun curbs and stepped-up efforts to improve school safety and mental health programs.