An act of vandalism at the SLAC National Accelerator Laboratory over the weekend that left thousands of protein samples thawing to their destruction has prompted an FBI investigation.

Rob Brown, the Menlo Park laboratory's communications director, said about 3,500 samples were destroyed on July 18 or 19 when someone removed them from their liquid-nitrogen storage units and left them thawing on the counter. The ruined samples were submitted to the laboratory by the Joint Center for Structural Genomics, a federally funded research institute based in San Diego County. Brown estimated the damage to be greater than $500,000.

Lab officials discovered the vandalism Monday morning, when SLAC employees arrived to work, Brown said. The vandals also left Post-It notes on the counter, but Brown couldn't say what, if anything, was written on these notes.

The San Mateo County Sheriff's Office investigated the scene before handing it off to the FBI, said Detective Sgt. Lisa Gibbons from the sheriff's office. Gibbons said this was the first incident of vandalism at SLAC that she could recall.

"It's not your garden-variety vandalism case," Gibbons said.

A spokesperson from the FBI's San Francisco office declined to comment on the investigation and the agency's possible involvement with it.

Brown said the laboratory has "many thousands" of frozen specimens in its Menlo Park facility. The ruined samples, he said, represent a very small fraction of the laboratory's collection.

The laboratory is home to the Stanford Synchotron Radiation Lightsource (SSRL), which uses intense beams of X-ray radiation to examine materials on a molecular level. The machine -- which resembles a ring with booths around it -- attracts a wide variety of visitors and researchers, Brown said.

"This SSRL is really used by people from all over the world," Brown said. "They don't just come here to study the samples, they also submit samples to us."

The protein samples are stored in tiny, teardrop-shaped containers with a tail end. The robot picks up each sample by the long end and transfers it to the light beam for examination, Brown said. These containers are stored in cassettes, with each cassette including about 100 samples.

Investigators were reviewing their laboratory's logs earlier this week to find who has been visiting the laboratory over the weekend, Brown said. He said authorities are trying to determine whether the vandal was someone within the laboratory or a visitor. He said it didn't appear as if anything was taken from the laboratory.

The Sand Hill Road facility already has two gateways a visitor must pass through, as well as additional security inside the building. But the incident has prompted a fresh look at the laboratory's security protocols.

"Overall security is something we're looking at now," Brown said.