At least four high schools across Massachusetts, including two in Boston, received near-simultaneous bomb threats on Wednesday afternoon, the most logged by state police this year, Boston.com reported. While some were isolated incidents, others appear to be connected, and the FBI has joined the investigation.

While threats received by four high schools and one preschool across the state were determined to be hoaxes, one case is still being investigated, police and fire officials said.

"We don't know if they were in any way linked," said a dispatcher for the Massachusetts State Police, who is leading the investigation.

The schools which received the bomb threats include Amherst Regional High School, Boston College High School, Boston English High School, Worcester North High School,  Doherty Middle School and a preschool connected to Natick High School, according to Reuters.

After a threat was made at Boston College High School in Dorchester just before 12:10 p.m., the school was placed into "safe mode" until police arrived, a spokesperson said. Boston police also swept and checked Boston English High School in Jamaica Plain, according to officials.

Classes and other activities returned to normal at both high schools about an hour later after everything was confirmed to be safe, Jennifer Tegan, the school's director of marketing and communication, said.

At Natick High School, all students were removed from the grounds while the Natick police assessed a threat received by telephone earlier in the morning. Amherst Regional High School's telephone threat came in at 12:20 p.m., according to a letter that administrators sent to students' parents, informing them about how students had been evacuated to a nearby middle school.

Additionally, North High School in Worcester also received a bomb threat, according to the city's police department, but after a search of the school, nothing was found.

Investigators are now looking into the possibility that some of the threats may have come from the same out-of-state phone number, according to an exclusive report by Fox25.

Recent months have seen a general increase of bomb threats in Massachusetts, some of which are believed to have come from a single source.

Last month, eight different high schools in Massachusetts were targeted, receiving bomb threats by email from an account belonging to someone who calls himself  "Jared the Terrorist," according to the state police.

Meanwhile, suspects can face up to 20 years in prison and a $50,000 fine if arrested for bomb threats in Massachusetts.