A former New York City high school teacher and his twin brother were charged Thursday with attempting to make explosives, in part by paying students to extract gunpowder from fireworks, officials said.

Christian and Tyler Toro, both 28 years old, had been gathering material to make a bomb since at least October, according to a criminal complaint filed in federal court in Manhattan. The U.S. Attorney’s Office, FBI and the New York police announced the charges Thursday evening.

New York Mayor Bill de Blasio said there is no evidence to suggest an ongoing threat to the city or its schools now that the two have been arrested, but he said agents are still investigating.

“We get daily reminders of the threats arrayed against us, but what we’re seeing here in this case already is, some good people stepped forward with information,” de Blasio said.

Christian Toro taught at a high school in Harlem but resigned shortly after a student called in a Dec. 4 bomb threat, according to the complaint. A student was arrested for making that threat, but the incident sparked a broader investigation into Christian Toro, officials said.

Shortly after the threat was made, Christian Toro resigned his teaching job. After the resignation, Tyler Toro returned his brother’s school-issued laptop, and a technician found a book about explosives on the computer. Authorities were alerted, but Christian Toro told them he hadn’t meant to download the manual, and he’d discovered it while researching the 2013 Boston Marathon bombings.

On Jan. 31, Christian Toro was arrested on charges of raping a minor, authorities said. On Wednesday, FBI agents interviewed several students at his former school and discovered the teacher had paid at least two of them $50 an hour to dismantle fireworks and retain the gunpowder, the complaint states.

With that new information, authorities searched his apartment and found bombmaking material, including 20 pounds of iron oxide, and other things that could be used to make a bomb — including metal spheres that could be used as projectiles, according to the complaint.

Agents also found a diary, the complaint said, which appeared to belong to Tyler Toro and contained statements including, “We are twin Toros strike us now, we will return with nano thermite,” according to court papers. Agents also found an index card on which was written: “Under the full moon the small ones will know terror.”

NYPD deputy commissioner John Miller said investigators are still working to understand “the full breadth of what the materials mean.”

Neither suspect was known to counterterrorism investigators prior to the probe that began in December, Miller said.

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