The FBI is investigating a wave of active shooter hoaxes at US universities which have triggered chaotic armed police responses as students return to campus for a new semester.
Since Thursday, nearly a dozen universities have been hit by swatting hoaxes - leading to panic as students, parents and faculty flee.
Authorities say swatting - making prank calls to emergency services to deploy a Special Weapons and Tactics team (Swat) - has grown in popularity.
In a statement issued in response to the latest wave of hoaxes, the FBI encouraged the public "to remain vigilant and immediately report anything they consider suspicious to law enforcement".
Villanova University, a Catholic university in Pennsylvania, has been hit by two separate threats in just four days. Student Ava Petrosky was singing in an orientation Mass on Thursday when people in the audience began to run.
