Six children are suspected of orchestrating a nationwide campaign of bomb threats against black colleges, law enforcement officials have said.
More than a dozen universities and places of worship have been targeted by hoax calls, which spiked on the first day of Black History Month on Tuesday.
The child suspects reportedly used sophisticated methods of technology to disguise the origin of their calls.
The FBI is investigating the incidents as racially motivated hate crimes.
Houses of worship were also targeted in the calls, but no explosives were found at any of the sites.
In a statement on Wednesday, the agency said that the FBI Joint Terrorism Task Forces was conducting the investigation with “the highest priority” and that more than 20 FBI field offices were involved.
“Although at this time no explosive devices have been found at any of the locations, the FBI takes all threats with the utmost seriousness, and we are committed to thoroughly and aggressively investigating these threats,” the FBI statement said.
The threats “are being investigated as racially or ethnically motivated violent extremism and hate crimes”, the FBI statement adds.
According to US media, six children have been identified as persons of interest in the case.
One unnamed official told NBC News that the group appeared to be “tech savvy”.
Investigators were searching their homes and conducting interviews, the Wall Street Journal reported, adding that it was not known if any arrests had been made.
More than a dozen historically black colleges and universities (HBCUs) received threatening calls on 1 February, the first day of Black History Month in the US, as well as the day beforehand.
Dozens of campuses and places of worship, including black-affiliated churches and Jewish synagogues, have received similar calls since the beginning of 2022.
The universities that received the threatening calls were forced to close doors and send out shelter-in-place alerts. Many moved to online learning as bomb squads combed campuses for suspicious objects.
In Daytona Beach, the caller claimed to be affiliated with a neo-Nazi group Atomwaffen Division and threatened to open fire in addition to the bomb threat, according to the local police chief.
“He advised that there were seven explosives that were disguised in duffel bags and backpacks and they were all C-4 [explosive],” said Police Chief Jakari Young.
“There was a secondary threat of an active shooter, an active shooting that was supposed to occur,” he continued.
White House Press Secretary Jen Psaki called the threats “disturbing” on Tuesday, adding that White House officials “take these threats seriously”.
“It is scary, it is horrifying, it is terrible that these students, these faculty, these institutions are feeling under threat,” she said.
“A lot of us feel like this is political,” a 19-year-old student of Spelman College in Atlanta told the New York Times.
The FBI has previously warned of a rise in white supremacist-fuelled domestic extremism.