Students in a wealthy Dallas enclave will find tighter security at their high school after several threats were received over the last week.

The intensified security Friday comes after a threatening note was found Thursday atop a hallway stairwell at Highland Park High School. It came one day after someone left a box of .22-caliber shells in a boys' bathroom. Another written threat was found in a bathroom Tuesday.

In a letter to parents, Principal Walter Kelly says hand-held metal detector wands will be available for searches if necessary. Thursday's letter tells parents and staff that school will be open regular hours.

Highland Park High School in January received three other hoax notes. A $7,500 reward has been offered for information leading to an arrest and conviction.