Update: Lockdown Ends at Mar Vista High School, IB Elementary

Updated at 5:30 p.m. May 1, 2012 to include information from San Diego County Sheriff's Department Lt. Marco Garmo and 5:30 a.m. May 2, 2012 to verify the school was cleared of any explosives.

A bomb threat called into Mar Vista High School Tuesday minutes before classes were scheduled to get out for the day forced sheriff deputies to lock down the campus. But students were allowed to leave campus about 3:30 p.m.

No explosives had been found by 5:30 p.m., authorities said. The school was later deemed clear of any explosives. Classes will resume Wednesday.

The caller said six bombs were placed across the campus, and he was armed on a nearby rooftop with a high-powered rifle with a scope, said Lt. Marco Garmo.

"We know a call came in directly to the school around 2:30 p.m.," said Manuel Rubio with the Sweetwater Union High School District. "It just hapens that our (SRO) School Resource Officer was on the scene and they advised a lockdown."

School Resource Officers are part of the San Diego County Sheriff's Department and are based at schools throughout the district.

Imperial Beach Elementary School and South Bay Union School District offices next to the high school were also shut down.

"As far as we know everything's OK," Rubio said around 3:30 p.m. "They just wanted to make sure and were just waiting to hear back from everybody the school site and the sheriff's department."

Calls were made to families to let them know about the incident, Rubio said.

Air support was provided by a San Diego Police Department helicopter, who stayed above the area for about an hour and cleared rooftops within a mile radius of the school campuses, Lt. Garmo said.

"We did establish communication with the caller through the principal's office and we received some information there, and we're still following up on that," he said. 

The San Diego and Coronado police departments assisted with crowd control, blocking off portions of Imperial Beach Boulevard and Elm Avenue and establishing a perimeter.

"Currently, Harbor Police Department with three bomb dogs and Sheriff's Bomb-Arson Unit are clearing the school for any devices," Garmo said. No bombs have been found.

The lockdown at the high school and the South Bay Union School District offices ended around 3:30 p.m. At that time, students were instructed to walk home or go to St. James Lutheran Church at the corner of 9th Street and Imperial Beach Boulevard if they were being picked up by their parents.

Imperial Beach Elementary School students were picked up at the front of the school on Imperial Beach Boulevard as well, the district's site said.

The sheriff's department instructed students to turn off their cell phones for a multitude of reasons, Lt. Garmo said. 

"A lot of times explosives can be set off by different frequencies, so that was something we wanted to take into consideration," he said. 

Phones were also shut off to avoid the spread of conflicting information and to avoid a frenzy amongst parents concerned for their children's safety.

"We want to control the information going out to make sure it's consistent, vetted and confirmed," he said.

Correction: The original version of this article stated that varsity baseball and softball games were scheduled to begin at the high school at 3 p.m. but the team's schedule was for games to be held at Castle Park High School.