"We always see an uptick in threats of mass violence targeting schools at the beginning of the school year," Special Agent in Charge Jason Hudson said.
The FBI said three arrests were made in connection to threats against Pasadena ISD schools, and one was made in connection to a threat made to Huntsville High School.
Hudson said the four individuals arrested have been charged with making a terroristic threat.
Pasadena ISD said in a statement to ABC13 that Memorial High School and Sam Rayburn High School were the schools threatened within its district.
The FBI says it can't share any information on the fourth threat it's investigating, adding that nobody has been charged in that case.
Hudson added that any threat made against schools will be taken seriously by the FBI and its law enforcement partners.
"This is not a laughing matter," he said. "We do not find this funny, and you will still be held responsible for your actions even if you don't have malicious intent."