OCEANSIDE (CNS) - A telephoned bomb threat received by a staffer at Oceanside High School prompted a lockdown at the campus Wednesday afternoon -- the third to occur in the San Diego area over a four-hour period -- while police searched the grounds.

An employee at the North County school got the menacing call shortly before 2 p.m. The voice, which sounded automated and computer-generated, stated that a rented moving truck loaded with explosives was going to be driven into "your building," Lt. Leonard Cosby said.

"The caller did not mention the high school or anyone by name," Cosby said.

Due to the vague nature of the threat and the fact that most students had already left for the day, administrators instituted a "modified lockdown," a precautionary status that allows for some supervised movement around the campus, said Steve Lombard, a spokesman for the Oceanside Unified School District.

Officers searched the grounds and buildings, finding nothing threatening or suspicious. The lockdown was lifted in the late afternoon, and police helped school officials monitor the release of the remaining students, Cosby said.

Late Wednesday morning, a pair of similar bomb threats received over a two-hour period disrupted classes and led to lockdowns at San Ysidro High School. Those calls also sounded machine-generated, authorities said.

In Chula Vista, meanwhile, reports that a man possibly armed with a gun was wandering around near Otay Elementary School prompted yet another lockdown about 12:30 p.m. A suspect was taken into custody for questioning at a home north of the campus in the mid-afternoon, at about the time that pupils were being released to their parents.