An unfounded threat caused school officials to place Plant High School on "modified lockdown" earlier today for about an hour, said Hillsborough schools spokeswoman Linda Cobbe.

The impetus for the move was an "unsubstantiated rumor" that stemmed from a parent who told school officials that students "overheard a threat" on Friday, Cobbe said.

The threat was not found to be true, Cobbe said.

A call to parents was scheduled to go out at 9:30 a.m. today, but a glitch in the system caused the call to go out around 1 p.m., Cobbe said.

School officials this weekend announced increased security would be evident at Hillsborough schools after Friday's mass shooting at a school in Newtown, Conn.